Multiple interviews and reimbursement from the same institution
It does sound to me like you should disclose that you are doing other interviews at that campus, particularly since they are likely to notice you back in the same building later in the week and wonder why you are there.  And, the department offering to pay your travel is probably going to wonder why you are spending the whole week there when your interview is just one day.  Finally, I imagine them knowing you're being courted by multiple departments might make you seem like a more attractive candidate.