An old school "biker leather" store, you know, the kinds that used to flourish in the 1960s and 1970s - and still exist in some places. The sort of place that carries sturdy leather jackets and some patched together from the cheapest Pakistani hides possible, for the poorest of bros. The ones with very sturdy hand-tooled leather belts, belt-mounted leather cigarette pack cases, studs and conchos a-plenty, a few half-helmets, a wall of helmet stickers ("Honk if you've never seen a gun fired from a moving motorcycle", etc.) Where you can find large belt buckles that include a Panhead motor and Confederate Flag motif, T-shirts that have questionable mottos ("For my next trick, I need a condom and a volunteer") alongside the giant, heavy but cheaply made metal rings that look like the Grim Reaper or the number 13, whose original purpose was to get around the brass knuckles ban. The kind of place that is often relegated to flea market and open air "rent a table" marts status. This sort of place might have a few of these bells tucked in on a small display stand next to the pin keepers (holds any kind of pin thoroughly in any kind of leather!) and these days, the accessories like the handlebar mount to hold a cell phone or drink caddy.
What I'm talking about is a small bell, maybe the size of the tip of your thumb to the first joint thereof. Tiny, but enough to be heard clearly when moved. It's usually metal colored, or black, and some of them have images like skulls or what have you cast into the surface of the metal. The originals were very small clapper bell type bells.
The original reason for these was pretty straightforward. Until laws and procedures caught up all you really needed to do to "launder" a stolen motorcycle was to source a set of stamps like the manufacturer uses, grind off the original serial number and stamp another set. (Intriguingly, these days some people use these to preserve the original stampings in order to keep their bikes on the right side of the law). Either that or simply buy a new set of engine cases, and replace that on the motorcycle, deeming it a "custom". These days if you bring in a motorcycle to be titled or registered you need receipts for every single nut and bolt on the machine, and there are now serial numbers on the frame and the engine and they must match. In more than a few places the police will simply confiscate and place a bike at auction that deviates from this, even if you have a legal title and valid reason for it (the cases were damaged, etc.) which is why people with older bikes or bikes that have had to have parts replaced avoid large rallies like Daytona or Sturgis.
Also, the general level of security on a motorcycle was pretty lax, because their theft was not presumed. One early modification on most Harleys was to junk the original ignition lock because if you had ANY Harley Davidson key you could pretty much walk up to a Harley and insert it and ride away as if it was yours. It's not as if it isn't that difficult to hot-wire a bike relatively quickly, but still.
So if you were at a bar, or a run, or a rally, and you heard a sudden bell ringing, you knew someone was moving a motorcycle somewhere, and that turned your attention to the person soon to be disappearing under a hail of blows and swung bike chains protesting that he was just looking at it, I swear. Other precautions were taken (having a designated person standing there looking at the bikes at all times, a job assigned in a motorcycle club to a prospect wishing to join) but the general idea was you were generally close enough to your bike to hear it jingle if someone messed with it, and given no two bells sound the same, you could possibly even pick out when it was your ride.
And, either as a cover story or just simply through a process of things changing over time, a sort of lore was built up around it. From "it's a sensible idea to put a bell on it to hear when someone moves it" comes "this will bring you good luck" and from that, it became a sort of amulet or good luck charm that people placed on their machines as a matter of habit. They became known as gremlin bells, from another piece of whimsy that built up.
A lot of early bikers were airmen in the armed forces, and many of them brought military traditions with them. Biker jackets are nothing but a civilian version of flight jackets adorned with the kind of patches and small medals that military people collect during their time in the force. Likewise, the military's concept of the "gremlin", a kind of ghost or malevolent elf that affects and damages mechanical devices you count on to survive also carried on in the biker world. (A famous Twilight Zone episode brought the gremlin to national consciousness with William Shatner seeing one tear up an airplane engine at 30,000 feet.)
Anyone who's dealt with a machine intimately, especially one as temperamental as an older Triumph, Indian, or Harley Davidson knows that each machine has a "personality". In between the design choices made by engineers and wear and tear patterns by present and previous owners, each vehicle has its own particular little set of temperaments. One might need a hotter spark plug, and another might need a certain "just touch" to shift into neutral. For people who rebuilt and rode their machines many many times over many many distances, they'd get to know where their machine excels, and the kind of things that bedevil their paricular ride (it keeps spinning the front exhaust tappet way up, and I've replaced it four times!).
So the joke came about that the gremlin bell was designed to ward off or otherwise annoy and get rid of "gremlins" that would attach themselves to a motorcycle and bedevil the owner. The further joke came about that potholes were the result of gremlins rapidly leaving a ringing machine and crashing hard into the asphalt.
But bikers are pretty fatalist anyway. Riding extremely fast on a machine that requires speed to stay upright and can't lock up its brakes like a car can means that the Grim Reaper and skulls are a constant motif in biker culture, never mind the reminder of how dangerous the pastime can be when attending a rally or going to the local parts shop and seeing that a funeral or charity run to pay medical bills has been set up for a fallen rider. Whereas most riders aren't particuarly religious, they do try to get any kind of edge or help they can get, even if they're the staunchest of athetists.
So it's pretty much standard to see, especially on older Harley Davidsons, a small bell ringing from a not-so-obvious place, attached by a small wire loop to a bracket or sometimes openly attached to a footpeg or voltage regulator mount. The practice is really an American biker thing, and a vintage one at that, so it's dying out. A combination of newer riders not knowing the tradition, bikes being better built and a lot safer, and the influx of sport and Japanese riders who wear brighter colored leathers and full face helmets who think, rightly, that a small bell would look ridiculous on a GSX-R have lead to a decline in the practice. But you can still see today in upscale Harley dealerships a small selection of bells in a jewelery display case, as some are wont to keep the gremlin bell going.
You're not supposed to buy your own - for best luck you are supposed to be gifted the bell somehow or acquire it as part of a motorcycle purchase whose previous owner had invested in such things. So if you know an old greybeard or even a young fella buying his first Sportster and are looking for something to get him, consider a tiny inexpensive bell. If he knows the tradition, he'll be delighted, and if not - well, you have quite the story to tell.