Ghostly Villains of Atlas Park

This morning I was doing some street sweeping with my tanker in AP when I came across some Hellions. As I approached them, they faded from sight and I lost my target lock. I shrugged my shoulders and continued around the corner where I found some Vahz.

When I had dispatched the Vahz, I turned around to see that the Hellions were back. When I approched them again, they again disappeared, but when I backed off, they reappeared. I managed to hit one with my Blackwand inherent power while backed off, and when he rushed me, he faded away again while his buddy still shot at me from range. The first Hellion must have lost aggro on me and walked back to his buddy, because he reappeared when the distance between us increased to some value.

I eventually dispatched the two of them from range, but then found the same thing happening with a group of Vahz and a cluster of Clockwork in the same little area. I ran a demorecord of the encounter with the Vahz and the Clockwork to show what was happening, but I'm not sure how to "post" it, or whatever.

I believe what you're seeing is the phasing tech in action. Those Hellions are not there for you, so when you get close the game decides that they shouldn't be there and phases them out. I've seen similar things myself.

It sounds like the area where the Arachnos attack. That area IS Hellions...right up until you do the mission for Aaron that turns that area into an Arachnos occupied spot. But until you reach that 'phasing' line as it were, all you'll see are the hellions.