Even though Dave has been confined to a comfortable but restricted indoor space for the last year or so, he has not lost his will to investigate. He thinks there is always more to be found. That's why you can often find him peeking behind the same curtains and underneath the same pieces of furniture he has already investigated dozens of times.
Although Dave knows he is a great investigator, rivaled only by the likes of Sherlock Holmes and Philip Marlowe, Dave also knows that he might have missed a clue the first time he looked somewhere, and he doesn't want to let hubris prevent him from learning something new. That's why he is snooping on the bathtub here, even though he has investigated it many times before. He knows that new circumstances might have altered the bathtub in some way, and he wants to know about anything unusual.