How much privacy is too much privacy in a relationship? At what point does 'privacy' start looking like secrecy? 👀

Give them the benefit of doubt for now.
If they are with you.

Maybe it’s nothing

In my experience privacy is a weird thing to gauge. The way I’d explain it is more trust or details. For example I don’t mind my partner checking my phone or location but I wouldn’t like them constantly checking my phone all the time.

How does a relationship even form if someone wants that much privacy to begin with.

Very fine line which has to be decided by the players

You could write a whole book on that. It would be better to narrow that down so answers can be given in a paragraph