Well I figured out how he got all the way up to the overflow.
I was moving 2 fish over from QT into the main tank last night. It was after dark and I had all the lights off except for the red ones (supposedly fish can't see reds so as far as they were concerned it was nighttime). With the red light I was able to mostly see what was going on in the tank. I noticed the pistol shrimp was coming WAY out of his burrow exploring, thinking he had the cover of darkness. Well apparently one of the other fish detected him, rapidly came toward him, and he shot across the tank and nearly back up to the overflow in a panicked retreat lol. I assume that's what happened. I'm going to have to keep a close eye on the filter floss and refugium from now on because it may not be a one-time thing!