If I went to a party and a man there would just in the course of normal conversation mention that he was "introduced" to Madeleine McCann, I would as soon as practicable gone to the hosts, and ask who this man is, and when I would found out as much info about him as possible, I would call police.  However, if it was 1am and I had couple of drinks, I might also wait till the morning. 

BUT, how unlikely is it that a total stranger would say something like that?  Unless, it is that sort of party where all the people present are into abusing kids one way or another - either actively or passively by watching videos of child porn.  Now, if that is the case, people do not tell each other exactly who they are, and asking for more info about another person present is also not a good idea.  Going to police would have been at enormous personal cost for this man, but he did, and has been arrested.