Why would someone say this to their girlfriend?

Im in a car club so i like to sit in the car and chill. Im literally right outside of my house. I've always sat in the car even before we met. Its just a comfort/therapy thing.

My boyfriend asked why I sat in the car for so long last night. I said you could have called me back but you didn't. If he's suspicious he will sit on the phone with me and thats fine with me. He said “if you sit out in the car that long again, I will move on and you won't know about it just like I had to find out this way.” Find out what? I didn't even do anything. And why make threats like that? Is it for control or could that be his way of saying he wants to?
Why would someone say this to their girlfriend?
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