People behaving purposefully nice, and not because they are your friends or they are or want to be on a date with you. Red flag, freaking red flag. Can happen for example at work. So difficult to notice, just like ass licking is hard to notice when it is done to you. It is before you even have a chance to notice what is going on, you get swept away by your ego being stroked, and totally forget to check in with the reality. I am not sure if this works this way with everyone, but for someone desperately craving validation and acceptance, hell yes, it does.

That is a huge manipulation trick.

People just don't do nice things for no reason. If they have no personal relation to you, they are either trying to prepare the ground for how they plan to behave later, they want to ass lick you, or they themselves have the problem of compulsively trying to fix the world. But if their trying to fix the world is not consistent - they are just looking for taking an advantage. And that can be sign of a profoundly screwed up personality, which should be avoided at all costs.