I find threads like this worrying actually.
I perpetuates the idea that we will be able ot spot criminals and that we can rely on our own brilliant good sense etc.
That is why so many people fall for scams etc, because they, subconsciously maybe, think that it woudln't happen to them because they would just 'know'
Obviously sometimes people are creepy/odd/ strange.
But many criminals, including some of the most dangerous, are very 'normal' people. This particularly applies to paedophiles who are often very good with children, which is why they (appear to) make excellent youth leaders/scout leaders etc, and why so many parents refuse to believe that that nice Mr X coudl have done that.
I knew a guy for 15 years. Mr ordinary, Mr nice guy next door.
He married a really close friend of mine. They were happily married for 12 years with kids.
then the police knocked on the door. To cut a long story short, he was convicted of rape of a 3 year old and possession of thousands of pornographic images of children. His wife had no idea, her world fell apart. He appeared to be an ordinary bloke. I had known him for years, there were no indicators, no clue.