I did and have very similar thoughts about next weeks episode. They showed Glen chaining up the back door (when Darryl was saying Meryl was sorry) so im thinking they filled the prison with walkers and are waiting to ambush, locking the Gov. and his lackeys inside the tombs.
That very well might be the case. Prisons are built to keep people IN, not out, so the group might use that to their advantage. Hide, lock a few doors behind the Woodbury group as they move through the prison, release walkers into their midst, and shoot any survivors as they come back out of the prison. That would be a pretty solid plan.