I was reading about the level "Euphorphobia" (yeah I know, I spelt it wrong) and some people were wondering why you did'nt find the leader on that level. Well, my theory is that, instead of dropping you right in front of him with a dozen guard's weapons pointed at your back, he dropped you off a little ways away. And when the fearless leader found out you were on board, he high tailed it out of there with some of his men. Considering you came in suddenly, not all the pfhor could run, or more likely they were a rearguard.
And about that rifle with the grenade rounds, I got the impression that it was left after a fight, picture this; A single human manages to escape from the phfor and is either armed or finds weapons. He's running down a hallway when a group of phfor come screaming torwards him, he emptys his last clip but in fear manages to not hit anything but the wall. While he's fumbling with the grenades, a phfor simply grabs him instead of killing him because they don't have time to fight, he drops the rifle and grenades, and they continue the retreat.
On second thought, that's a bit of a stretch...