Walkie talkie motion sensor?
In detail, the idea would be while playing paintball or something similar, you could plant it somewhere... once someone went passed it, it would use the alert function on the walkie (I don't know what the proper name is for that button, but it's the one that makes the obnoxious noise when you press it). This would allow everyone on your team to know when the other team had passed that point.
So, how would I tackle a project like this? As an electronics newbie am I in over my head, or is this something that I could handle with a little messing around? Ideas?