Just now we were playing on Crossfire, only five of us. Every now and then I'll put up C4 in a spot where I know people like to camp. On Crossfire the best one is the two story building on the top of the hill. So I tossed a C4 up on the ceiling there and went about my way, walked all the down to the bottom, and then back up again - didn't see a soul. Suddenly I was pinned down and I could tell someone was in that spot... I popped the C4 and killed them.
In other words, I set the C4 charge, walked around the entire map, noticed someone was up there, set off the charge and killed them. Sweet.