Anyone Else Seen This?

Last night I was farming iron on Mars in my usual spot just off the dropzone for the patrol. I had maybe made one circuit around the area and was clearing the area in front of me when I noticed I was taking fire from behind me. Thinking I already cleared that area I turned around and nothing was there. But after standing and facing in that direction for a minute I could see fire being directed to me from a wide open space with no one else there. It was so weird I went and got my son to look at it to make sure I wasn't seeing things. I took out Zombie Apocalypse and emptied an entire clip in to the area with no effect...the fire would start and stop as I went in and out of visual range. Now, I have seen them mob the area when several people are farming but never this...anyone?