I invaded an invisible guy in the Archives yesterday. Searched every inch. It was bizarre in that no enemies were dead, the serpent soldier you kill to get the cell key was still alive, and the doors holding in the Pisaca (Pisacas?) were closed. He was one of few people playing though and I ended up invading him 5 or 6 times.
I had a pretty fun kill that made up for it, though. I chased a sinner through the Archives and he slid down the ladder behind the sliding bookcase. When I got to the top of the ladder, I looked down to see him standing behind the ladder at the bottom, thinking he was pretty clever. I thought so too for about half a second before I had an idea. I applied Sunlight blade to my obsidian greatsword, leapt over to the ledge across from the ladder, and then rolled towards him and plunged. I died, of course, but thankfully did enough damage to take him out.