There is a top on the tankUnless you saw the clowns being rude (and they can be!), I'd think it unlikely that the goby would have jumped. However, they are small, and it wouldn't take long for a healthy tank to process the remains.
Any reason you haven't put a top on? I tend to favor them for all tanks, but particularly for SW since swings in salinity have been shown to be SO detrimental to corals.