Most of us have a decent judge of character and if there's something about another person (especially someone who has made their way into your family or workplace) that feels a little off, you're probably right. Many people can be too blinded by love or charming personalities to see even the most ferocious of blazing, blaring red flags.
Safety is critical, though, and just because a creepy vibe from a person isn't an unaccounted for backpack on public transportation you should still say something if you see something! When a recent Reddit user asked, "Have you ever (correctly) had a gut-feeling to dislike someone who, on the surface, was a cool person that everybody else liked?" people were ready to share their horror stories.
I never trusted my ex bf's uncle. I just knew he was trouble and had issues with him visiting or my ex visiting him.
He was arrested in Florida last year for production and selling meth and hosting a prostitution ring out of his home. Later learned the ex partook in all the activities at his uncle's home. Apparently my gut was right about the uncle, but not about my ex.