Missing Urchin

[HarleyC]

Hi,
Completed the tank relocation last Friday and we noticed then that one of the pin cushion urchins wasn't looking particularly healthy. But over the next few days he moved around a fair bit.
This morning I noticed he was about a third of his usual size and now I can't see him anywhere. Am I to assume he has gone to the big tank in the sky?
I have 160lb of live rock and don't want to tear it all apart searching for the corpse either.
Will there be any ramifications to just leaving it be?
Thanks