I keep hoping I see a definitive conclusion as to whether Apple has a specific way of knowing if you have upgraded the RAM and then reverted to bring in for warranty work. Even if not, that's a crummy way to have to live.
Pretty sure I saw paint on screws, remove the screws, paint goes off, so yeah, they can see you opened it.
If you are in Europe though it should not be a problem, your warranty isn't voided by replacing the RAM.