Sorry - but that isn't true - suprise scav drops after you have started the mission still happen. I have been surprised on several occassions mid 'exploration' after completing the mission.
I had them at the start of the mission and then they even got reinforcements while i was faffing around trying to figure out how to deal with them after nailing a straggler without alerting the other scavs at the site (that's on my epic alt running in g2-g3 store equipment)