I've never seen an anemone eat another one (possible I guess), but it's not unusual for an anemone to just disappear in a reef tank. Sometimes they reappear, sometimes they're never seen again...
FWIW, one of my condys broke loose a long time ago and got sucked into an unprotected PH, i caught it soon after, but if it had been in there for a while, I'm sure it would have eventually been destroyed and chopped up.
Do you have an overflow and sump? Check there. My condy took the funhouse ride a couple of times. I'm still not sure how it got over the teeth in my overflow. That went on until I found it too late in my sump anchored to my titanium heater.
yeah i had a BTA get sucked into a PH...I witnessed, although already too late at that point, the impeller of the PH rip up and spit out pieces of the anemone..
I had a condy squeeze itself into the little slots at the bottom of the overflow and re expand in the inner wall, it was there for 4 days or so before it came out