The police apparently found his persona quite charming, a plausible reason to let him go.
Times, Sunday Times
Can't find a plausible reason for your character to explain his motives?
Times, Sunday Times
There's another more plausible reason though.
The Sun
The only plausible reason was age.
Times, Sunday Times
She also said that she wanted the noise to stop, and the most plausible reason for her leaving was surely the rising frequency of her trance episodes.
The Times Literary Supplement