When I had my betta, Sir Edward the third, I purchased 3 ghost shrimp (the three muskateers) because the wal-mart tank mate tag said bettas get along with ghost shrimp. Well, my betta was in a large plant vase (the ones you buy for bettas and put the plants in them.. i never put a plant in it tho)
Anyways, one day I was looking at my ghost shrimp and only counted two.. so I asked my friend to see if she could find the third one.. we looked and looked and looked and she jokingly said "maybe he got out." I looked behind the vase and sure enough the ghost shrimp commited suicide and was dead on my table. The next day one of them was missing a head and the third one was no where to be found.
So much for Sir Edward and the 3 Muskateers!
Also, my friends bought their bettas ghost shrimp and they said theire ghost shrimp just disappeared.
Timanator- you might want to look under the gravel if you don't have any hiding spots for them.