Grant is unlikely to be armed. He rarely carried his sword, except for ceremonial occassions, and did not, as far as I can recall, have a personal side-arm (it was not required by regulation) during the war. And, even as a general, he was not habitually accompanied by a bodyguard or armed escort. So, there may well be no military sentry present.
There was one present anyway, but he popped off for a drink or something. As I understand it, you could get away with taking such liberties with Lincoln, but not with Grant. So he'd presumably stay at his post.