Tonight, down by the Sharkhead docks, I saw some random guy and his bots fighting
nothing.
I stood and watched for a little while and he asked me for an SK (he was a lvl 12 and the invisible mobs he was fighting were, apparently, lvl 22s). So I SK'ed him and asked him to explain what the hell was going on.
It seems every night, after the Scrapyard Ghost spawn disappears, his huuuge mobs stick around. They gather around mission doors (like sewers and and trucks) but they're invisible. My lvl 12 companion couldn't see these mobs but he could set his bots to auto agro and target the mobs through his bots. I also couldn't see the mobs, but I could target them through his bots as well. He had a dark cloud AOE thing that made "seeing" the mobs a little easier.
I got a crapload of XP off invisible enemies that couldn't fight back because technically they weren't there.
Screencap -- click for larger!

So if you're on CoV at roughly 4am EST, check out the Sharkhead Isle docks for some easy XP.
