A remarkably similar experience happened to myself as well. It's scary how close the above story resembles my own.
I worked for CompUSA for a year in the Business Sales department. It wasn't a BAD place to work, but I wouldn't want to make a career out of it. (I did enjoy a certain recognition as being the store's resident Linux guy. "Can you tell me about Linux?" "Sure. Go to Business Sales, and ask for Chris.")
Anyway, it was a weeknight, and I had gotten off of work at around 10, and everyone else had stuff to do. I went to the break room to clock out, and I noticed the TV. Star Trek: Voyager was on, so having nothing better to do (and looking forward to an hour drive home), I sat down and watched it.
After the episode was over, I turned the TV off and walked out of the break room. Walking across the store, I noticed the front gate was pulled down.
Not a good thing.
Seeing this, I jogged to the back of the store to see if anyone was back there by some chance. Nope.
This is where it got fun. While in the back administration area, I decided It would probably be a good idea to call someone to get me out. Just outside of business sales, there is a phone for customers to use. I started to run from the back of the store to the phone, with the intent to call a manager.
BEEEOOOO - BEEEOOOO - BEEEOOOO - BEEEOOOO...
Of course, genius me decides to run right across the motion sensor's line of sight. After hearing the alarm, I changed my course of action mid-stride. I picked up the phone and promptly dialed 911. Not because I was scared, but because I didn't want a shitload of police showing up pointing guns at me.
"911, what's your emergency?"
"Hi. My name's Chris, and I'm locked inside my CompUSA. I just tripped the alarm, so I figured I'd call you guys so you don't send SWAT or some shit. It's just me in here."
"Yeah, we registered the alarm. There's a police cruiser on the way, so we'll fill them in on the situation."
She tries a few times to get in touch with the alarm company, and finally succeeds. From there, the alarm company gets in touch with Ben, the operations manager. He's got a bit of a drive, so I'm stuck with a bit of a wait. I walk over to the front gate, and sit down to wait for the police. They finally show up, and they get a smile out of the situation. Well, I'm glad to brighten your evening, officers. I was laughing about it too. Why not? It was funny as hell!
During my sit, The alarm finally silenced. I wanted to call home just to let them know I'll be late, and why. While walking over to the phone, the alarm went off again. I went back to the front gate, and one of the cops said "Just stay right there!" He was stern, but trying desperately to keep from laughing.
Eventually, Ben showed up and saved the day. While walking up to the door, he looked at me and said "Chris!" shaking his head. I explained to him that it wasn't my fault this time, and I was right. The closing manager is supposed to do a walk of the store before locking up. When Voyager ended, it was still early by employee time.