All lock codes should be cracked and given out free for a very good reason.
the device does not go on air until the lock code is keyed in and if everyone reports a stolen device along with the emei number it is then flagged which in turn truly bricks the device stopping any usage whatsoever as it will never turn on again.

So all this secrecy regarding how it is done is useless as it is free to get anytime on the net anyhow.

Why bother asking for proof of purchase, give them what they want !!!.