Update on this: It's not just when I leave. When anyone leaves, it spawns Screamers on top of the remaining survivors. There's only about six of us playing. It must be that the heat is divvied up amongst the survivors, and then when someone leaves, it recalculates the heat amongst the remaining players. Since we have so few players, the recalculation is drastic.
For example (pardon my made up numbers): Let's say 100 heat attracts a Screamer. If the total heat on the map was 540, but there were 6 players, that's only 90 heat per player. As soon as a player leaves, 540/5=>108, sudd