Excellent point! I hadn't thought about that. Otherwise they'd have to have someone check the room every so often. Speaking of which, do we think that Quirrell left some of the potion in the bottle before going through the flames? Wouldn't have drinking the whole thing have been better? Or do we think it refilled itself? If it refilled itself, then wouldn't it have been smart for Hermione to wait for it to refill, then go through and help Harry?

Unrelated note and going back to the beginning of the book... Why isn't there a disguised witch or wizard helping muggleborns onto the platform? Or do we think not telling Harry how to get onto the platform was just an oversight by Hagrid and they usually make sure to tell them how to get to the train?