Unless there is some evidence the police have that we don't know about, that they had enough to pursue a mental hold on Matrice "imminent threat of harm to self or others", NO, they could NOT legally hold her because she was acting strange.
The arrest & booking was for an offense (not mental status). Once police processed her for that offense she was free to go unless they had the legally defined mental criteria allowing them to hold her against her will. The police invited her to stay and offered her a place to stay, but they could not legally compel her to stay.
Please, if you disagree, read my post directly above. The system can be changed if enough people let their disagreement be known.
Believe me, I read your post numerous times before responding. I fail to understand how it applies to Mitrice's case.
I guarantee, unless someone who was at that station spills the beans, we will NEVER know exactly what occurred there that night. I can see them keeping her car - though that is even a stretch, considering they didn't arrest her on a moving violation. However, if you can explain why they let her go in the middle of the night without her purse and phone, I would LOVE to hear it.
Apples and oranges, IMO. And the bottom line is, this young woman has been missing ever since. That is on their heads, as far as I'm concerned. Regardless of her mental state.