Why didn't Snape put Lupin under more pressure about the Marauder's map?

When he confiscated the Map from Harry and tried to find out what it was, the parchment started insulting him and showed him the nicknames of the Marauders. He clearly knew their identity that's why he called Lupin to his office.

Now Lupin denies everything of course and Snape doesn't know that's a map but why does he let Lupin walking off with it without confrontation? I know Harry is there as witness so he probably doesn't want that, but I would just send Harry out and make Lupin explain why such a suspicious artifact can be in Harry's possession which also has a connection to the mass murderer Sirius.