I always went up behind people like you and semtexed your back then shot you with a shotgun.
I didn't go behind them. If they were stagnant, I took the shot, but I went gung-ho into the battlefield, aim it a tad too high, DROP SHOT, body shot and that fantastic emblem in the killfeed, filling me with joy and validation.
Scavenger is glorious. There is no class it won't work with.
First time I did that to someone in MW2, the grenade made a direct impacts on the other player's groin as a final kill.
I don't even remember the first time. For me, that game feels like a decade ago. I've had a few semtex groin sticks that I can remember.
Semtex and noob tubes were how I screwed around in MW2. Quick-scoping was my constant challenge I could never exactly master. Marathon/Lightweight/Spas was like running GodMode.exe.