What are signs to you that someone is dangerous? Not in a physical sense but in other ways?

For me, the biggest red flag for someone dangerous is someone who completely flies under the danger radar. The person who gives up NOTHING of what they are capable of, or what sort of danger they represent to me.

Going through other answers, I think I can explain better.

If someone is manipulative, I recognize that I may be seeing and hearing what they think I want to see and hear, and thus, be on guard for any sort of jockeying they are trying.

If they ask a ton of questions (fishing for info), I recognize to look for relevance to them or usefulness of the information (what does having this knowledge DO for you?), to further analyze motivations and potential issues.

If they bring out the bad in me... (for me, that's an interesting beast to open the cage and let out-- I keep it locked up for a reason... so...good luck with that...! Just a helping hand, if you are bound and determined to be this person-- do NOT show any sign of fear or weakness. Unlike a cat, it doesn't PLAY with it's prey, and the second you show fear or weakness, you just put on the shirt that says, "PREY" in bold giant neon letters...) I recognize that I have a kindred spirit running with me, and our devils shouldn't be allowed to play with each other. At least, not in public.

So, with each of these examples, they are doing something that gives me notice to be on guard. It's the one that doesn't give me notice I am concerned with... for they are playing things so close to the chest, I can't get a read.

THAT is dangerous. ;)