Interesting, once you succeed with the first case then you seem very motivated to catch more other users who might just change their password for security, change their posting habits just to qualify for the campaign or something. I can't jump to conclusions for all the evidence you said, maybe he has changed hands and is sold but maybe he just started something new to earn a few dollars from the signature campaign.

The problem is, you have to give him chance to speak up and prove ownership of his account for while, that can be done by signing message from the bitcoin address he once used.

You can expect him to sign messages from this address: