Well how do they do that?
How do they legitimate themselves?
Are they for every airline?

First I would ask for which airline they work for and then tell them I am not flying with any of those.....no matter if it was true or not....( mistakes happen... I thougt I was flying Timbi-Air )

You do not have to talk to strangers ;-)

When I am asked is this your hand-luggage - I reply no! And keep walking....