On groundstays with less than 60 minutes planned hydraulics and fuel pumps (main, not center) have to stay on as per Boeing recommendation (737 classic and NG). Our engineers know that and they are not allowed to just walk up to the aircraft and stick a screwdriver into parts without visiting us chaps in the front office first and make sure everything is as they want it and we are briefed as how systems have to be configured during whatever maintenance they do. Otherwise the risk that we don't even notice they're there is way too big.