Yes, I do realise that some infections are just chronic, but it is relatively rare for them to go on for months with no other signs or symptoms. I do agree about dehydration, but would add that this behaviour is over 'months'.
I also take the point that it is often difficult to know whether the problem is dementia or something really happening, but there can be clues. Also 'spying' is a fairly common delusion, but quite rare in reality.
A doctor / specialist nurse or social worker have the training to look for the clues, and would routinely screen for infection (especially urine infection). That's why it can be helpful to give them the information, even if they can't discuss it.
I also think it likely that the 'warden' may know about this, and be monitoring the situation. OP wouldn't know, because the friend wouldn't tell her, and anyone dealing with it couldn't tell her! That's another reason for speaking to the 'warden'.