The photos have surfaced on several websites ... showing Heidi on vacation in the south of France with her bodyguard boyfriend ... sans bikini top.

There's just one problem ... it's unclear which French magazine published the original pics.

Sources close to Heidi tell TMZ ... the supermodel's camp is on the hunt for the source publication ... claiming her right to privacy was violated because she was on a private and secluded beach at the time the pics were taken.

Sound familiar? We're told Heidi is comparing her situation to that of Kate Middleton ... who also believes she was the victim of privacy invasion when a paparazzo snapped topless pics of her while she was sunbathing at a private chateau in France.

We're told ... just like Kate, Heidi wants to sue someone's ass ... but realizes she might have better luck if she waits for Kate to blaze the legal trail.