A quick followup to this..it appears that my pistol shrimp probably had a tunnel collapse on him, since I haven't seen him since I found the goby that jumped. From what I've read online, this isn't an entirely uncommon occurrence, unfortunately.
Either way, both have been replaced, and I picked up a nice ORA ponape birdsnest frag as well (couldn't pass it up for 40% off during a Memorial Day sale at my LFS).