Similar story - a member of my group went through security at LAX with no problems, but was stopped at Heathrow because of something suspicious in her handbag. We emptied everything out of the bag, re-scanned it, and the object was still detected.

After feeling around in the bag we discovered some fabric cutting shears had apparently been accidentally been sewn into the lining by someone on the factory line (they looked something like this if I remember correctly: http://imgur.com/ZKlsjJw).

We just cut them out, handed them over and went on our way. Was a really strange experience though.