digging in the night?

Ive not added anything to my tank for over a month and I woke this morning to find that something had dug in the sand bed all the way to the floor of the tank? And there where 2 small pieces of rock rubble at the bottom of the hole that looked to have been placed there?

What could have done this? I have


Bi-color angel
yellow tail damsel
percula
and yellow watchman goby
various hermits
4 turbo snails

+1

I'd bet that if you keep watching, the digger will be revealed.
Improvements in diet can lead to changes in behavior, mostly in a positive healthy way. Let me know when you need more spirulina enriched BS.

The other day I noticed a small hole in the sand of one of my Refugium tanks, surrounded by a circular mound of excavated sand. Could it be a small Pistol Shrimp or Mantis Shrimp tunnel? I watched for a few days to see if a shrimp would emerge. Nothing. So I poked it with a stick. The "tunnel" turned out to be nothing more than a foxhole under a piece of crushed coral. Two large Amphipods scurried away.