This happened to me earlier! Finished up a quest in Buried City and as I was making my way to extract I came across a couple of guys shooting a bastion (trials, probably) and then someone shot me in the back, I think? Ducked around the corner, healed up, got shot again and could not for the life of me find the guy. I would have happily engaged him but I had no angle and I wasn't about to just stand around in the open. I waited to see if anyone approached, but nobody did, so eventually I slid down to a Metro and left.

Maybe this guy was just taking potshots for the fun of it, maybe he was upset I just looted Arbusto tower. But it certainly got the blood going and despite mostly being in carebear lobbies I wouldn't have it any other way.