Very much agrees with Froggy, I keep metamorphs in large, naturalistic tubs with loads of leaf litter and associated fauna for food. Which has always served me well, and one or two individuals out of maybe 20 tend to disappear without a trace every year.
I did, however lost more than that one year, and started wondering why it was the case, did a bit of exploratory rooting around, hoping to find some of the missing morphs... only to be met by a very fat centipede! I am still using this methods, but am constantly on the lookout for small centipedes, and remove them for the set-ups before they have the chance to become larger ones!!!!