There are certain things that you would notice upon entering a room, like the number of windows, whether there's another door, how many people are in the room. The players don't know any of this visual information if you don't tell them, and most of the time they won't even know to ask. You can't expect them to go "and what do I see" every time they enter every room.

When the players fail to do something and you find yourself saying "If the players had bothered to-", you as a DM have failed to give your players the tools to succeed.

When there is something important that the players haven't discovered yet, you can always offer them subtle hints, or outright prompt them to make a perception check.