I have a school of 7 zebra danios. about 2 weeks ago, my son found one of them on the floor dead. It had jumped out through a hole where a filter used to be. My bad. I counter the danios - 5. I counted again and again. Only 5. It's a 55G tank with a big hollow log thing, and no other place to hide. So, I use th eopportunity to pull out the log and clean out the pleco poop that had undoubtedly collected in it. I washed it out well in the sink. No daino for sure. I checked the filter (between the pad and the front of the filter too, no danio. It was my son's favorite one too - it was a runt, smaller than the others. I checked back every now and again to see if it reappeared. One day, about 2 weeks later, it was back! My son had found it. Sure enough, 6 danios. Oddly though, it swims on the bottom of the tank now, not the top with the others. It weaves through the plants, around the log, etc, just as if it were on the top. The ones from the top go down to play with it now and again. another case of the alien fish snatchers.
Hopefully you're as lucky, but probably not with an rts.