Praise be to Allaah.
The basic principle is that we should think well of another Muslim, unless he does many things that indicate that we should not think well of him. This is what is known as circumstantial evidence or corroborative evidence. If this happens in this person's case, then there is no harm in being cautious around him, because he is the one who has brought it upon himself by his own suspicious actions. And Allaah is the source of strength.