I’m presently moving across the country, driving a big Penske rental truck with all my stuff in the back. I keep getting alerts about AirTags getting “left behind” at various places when I know they’re in the back of the truck. They seem to be getting detected when we stop by my (or someone else’s) phone, but when we’re moving, being in the back of the truck, they’re not close enough to my phone for it to pick them up. Since they’re not moving with me, FindMy thinks they’ve gotten left behind.
I have the same issue with my new Airpods Pro 2. Maybe it’s an issue with an update of “Find My” process? 
I have this occasionally with Airpods when driving. The Airpods are in my pocket right next to my iphone, but I still get an alert on my watch that they were “Last seen” in such and such a place.