It is very possible the shrimp crawled out of the breeding net they were in. They can be out of water for a very short period of time. I've seen my amano shrimp climbing along the outer sides of my breeding net, breaking the water's surface.
+1 I had to install walls onto the side of my 10 to keep my RCS from climbing out. Breeding trap wouldn't present much of a challenge for a determined shrimp lol.