They have ways of knowing when a user opens the unit, so don't crack it open just because there are no seals on the outside.

That's very true - there are a number of hints that a player has been opened. But sometimes it's much more obvious:

Some time ago, I had a player in for a fairly minor problem - in his emails the guy was quite insistent that he'd never opened the player and that the seal on the outside of the player had just broken in normal use.

Once the player arrived, it was quite clear that he'd opened it - the lid tabs were bent out of shape and had plier marks all over them.