During one of my missions into a level 27 Large Park in Brookvale East, when I was being swarmed by large numbers of infected I pulled out my medic who was low on health to ensure he would survive. As soon as he was away from any infected I sat him down and he started patching himself up, only to receive damage from an unknown source, I pulled him away and that's when the rest of my survivors started shooting at this invisible assailant, exp. was received from killing it.
The second encounter was with my engineer when I was pulling my survivors to the evac. zone only to have her start attacking and dealing damage the air. Exp. was given as well.