I moved trireme up from the south, found out it was a dead end, went to go back down and found that I was blocked by ocean tiles. Can tiles change or something? It's currently just sitting there, trapped.
EDIT: It looks like I did have an open borders agreement with Rome end, which must have put the trireme in the position. Disappointing, I was hoping for it to be some kind of prank from the ghost of Dennis Ritchie. Oh well, mystery solved, thanks for the input.