Nope. Radar irrelevant. Heavy spawns on same spot each game the enemy doesn't have to be on the radar to hit you with a rocket. Obviously this is map dependant. But to illustrate I got a triple on pantheon trials this weekend by picking up low heavy and firing blindly into the waterfall.

Eeh, not much good my radar is gonna do me as I get sniped or supered. Waiting for team mates means waiting for the enemy too.