Well, a feeling of trust, at least.

I suppose that's one way of putting it. I tend to think you can only be betrayed by someone if you trusted them to some extent.

Not true
People I didn't know did things behind my back. Was costly

Not necessarily, it will just hurt more.

No. You can feel betrayed on the basis of any relationship, regardless of "feelings for" the person.

For instance, we have been betrayed by fellow-citizens who sold or gave away our country's secrets, even if we didn't know them.

exactlyyy but I see tht stupid feeling of being hurt just have no particular reason
I don't know people but something will happen behind m back and unbelievably I'll feel tht hurt
all I can say is I'm dumb