I have a pair of zebra bar dartfish. Also very shy. Stay in the back rockwork except when feeding. One always come out first and the other slowly follows him.
One was always getting into my overflow and down to the filter sock in my sump. Even with a eurobraced acrylic tank that pretty much covered all around the overflow box’s teeth. Once I got a lid over the actual overflow box, he stopped escaping. So must have slid through teeth where it had open space to get through.