Hello and Welcome to R2R! I had three fish missing last week and all of them were in 1 of the 2 overflows. Not sure how it happened but it has been over a year since a previous fish did the same!
Have faith on that shrimp and goby pair they tend to go digging vacations and disappear for some time when expanding! You may see them yet! Especially if your hearing clicking at night. Welcome to the community