Well I thought it was implied that he watches Lana in her room because it immediately opens on to Lana's bedroom. And if, as he says, the tapes are only checked in an emergency, then why would one of the random guest bedrooms be the first place it opens up too?

And yep, their relationship is a bit creepy and strange lol. I remember Lana also had a teasing remark about Clark and his telescope when he used to check her out at her house. Kind of a strange girl :P