Ha ha! I get emails from a former employer, who looks at my skillset and seems to think I'm a good match ;) They don't seem to have noticed I used to work for them, or else surely they'd actually say "would you like to come back?" . My theory is the recruiters are having performance measured by volume of approaches they've done, so there's an incentive to contact anyone who's a reasonably plausible lead.

Used to happen to me too!