Why would agents think that a number written on your phone is the PIN? That would only make sense if it was a communally-used device, not a personal one. Also, no one would put sensitive info on a devices that has the PIN affixed to it.

I suppose it's possible someone might enter it without thinking, but the odds seem low. Also seems risky to put a self-destruct PIN on your device, lest a friend (or enemy) enter it by accident or as a prank.