Had similer experiance with BA MAN - LHR - BOS 3 years ago (have only flown BA once since). Flight MAN - LHR delayed by highwinds at LHR. On arrival transferred by mini bus to T4 up steps onto airbridge quick welcome on board, door shut, and push back underway while stowing carry on bag with clothes in. You already know that there is no way your hold bag - (with samples) got on your flight but with frquancy of service - 2 flights a day at that time you figure next plane will have your bag - wrong 6 flights later bags show up. BA baggage tracker said still in LHR when in reception Hilton Boston !. Do not get me onto the times when bags bound to MAN on the shuttle missed the connection at LHR !. I am sure I was on first name terms with courier !.
Intrestingly since moving carrier and now changing at Frankfurt or Dubai have not had one incident as above - probably broken my good luck by saying that though !.