Do enemies respawn infinitely?

I've just started the game and I'm 3 or 4 missions in. I'm doing a mission where the bonus objective is to find 4 relics.

I started wandering around to finf them and stumbled on an enemy camp full of zerg and buildings. I killed all the enemies, destroyed all the buildings but I noticed that one of them came back.

Do enemies on the map continue to respawn or was this a one off? If so, is there any actual benefit to killing them or is it better to get in, complete objectives and get out?

Thanks