Does an apple watch automatically unpair with an iphone after a while of it being unused or do you have to manually unpair it?

My apple watch has been missing for a while. It went missing with family over and it was last seen in my room. So it either is lost somewhere in my room (unlikely as I've checked every crevice multiple times) or it was stolen by one of my cousins. Every now and then I get the urge to look for it, give up, then a few days later get the urge again. I have a new phone but it has all of my old information so I wanted to see if I could go on the apple watch app and check once again if I can try to track it. However, it said that "You re using an iPhone that has been unpaired from your Apple Watch." Does this mean that someone unpaired it themselves or it's just been inactive for so long. I have no clue about this stuff. And if anyone has any ideas that would help me track it despite it been missing for a while, I d love to know.

It stays connected as long as the bluetooth connection to the phone is active. If the phone and/or watch's battery dies then the connection is lost but would be restored once both are able to have power & connect.

To help find it you could use Find My iPhone, which also could search for the Watch & ping it if need be.