It seems likely that the person who "snitched" on Magneto was this guy who saw him extend his hand and then look around suspiciously.
As to what motivated him to talk to the police, we can't really say but the general consensus seems to be that Magneto is a vicious murderer and would-be assassin of world leaders rather than a misunderstood advocate for mutant rights.
Police Chief: Nobody in this town really knows you.
Magneto: Yes you do. I am Henryk Gurszky. Jakob, I've had dinner in your home...
Jakob: And you were lying the whole time. I brought a killer into my house.
The news article held by the policeman talks about Magneto being an existential threat to the entire world.
"M-Day. 10 years Ago. The day on which which the mutants saved the world".