Learn English – What would you call a person who farts a lot

Blameshifter fits the bill.

blameshifting

Part of Speech: n

Definition: the act of transferring responsibility for an error or problem to another; also written blame shifting

_{Dictionary.com's 21st Century Lexicon}

I think the best way to refer to them (politely) is guarded. (Cautious, reserved, suspicious - from various dictionaries.)