I assume that police cells aren't fitted with undetectable evidence-disposal outlets.
I think that an episode of a moderately authentic British police drama "The Bill" (slang for police) featured a man arrested drunk who was, I don't know, just shut in a cell to sober up. The prisoner then claimed to have had money stolen by a young police officer. Exactly how what follows arose I don't remember, but it turned out that the prisoner had stuffed his money down the cell toilet, which he then used, copiously. The young police officer then was told off about proper handling of a prisoner and their personal property.
That does imply that a prisoner normally wouldn't get to take their evidence into a cell to dispose of it, anyway. The case we're talking about would have been an exception.