A feature of GAMES Magazine,
originally at odd intervals but lately twice a year, one of these always in the April Fool
ish April issue.
Unlike the normal GAMES contest which asks you to either (a) solve some puzzle and send in the answer, with winners to be chosen by random draw, or (b) play some sort of solitaire game (often a word game) to achieve the best score, the hidden contest is (of course) hidden in the issue somehow, and the goal is to find it, follow the simple instructions, and send in an entry. There are no grand prizes awarded in hidden contests, but some number of winners are drawn at random and win GAMES T-Shirts.
The hidden instructions basically tell you that you need to include some little text or trinket with your entry, such as a limerick or a paper clip or an origami figure, to show you've found the hidden contest. Originally the hidden contests were usually hidden along with the full instructions to enter, but lately the entry instructions often appear at the top of the letters to the editor and the hidden part is only the thing you have to send with your entry.