I think perhaps he was surprised by the surprise party because at that moment he and Lois were, uh, otherwise *occupied*. Clark's superpowers don't seem to normally function when he is concentrating on Lois. He becomes totally absorbed by her. Same thing with 'Sex, Lies, & Videotape', why didn't he sense that photographer trying to get a picture of Superman with Lois? Maybe because he was too preoccupied.
Just my take on that.
