Hidden phone

The obvious one would be to track the car's movements.

Smash the phone to bits and leave it on his pillow.

or just turn it off??

Chuck it in a bin

Gaffer tape it to a bin truck..

Put in in your outside on bin day

Destroy it and never admit to knowing about it. Then make sure he has no further access to your car

Gaffer tape it to a bin truck..

Yes, do this!!

I think that's against the law to track you. Hand it in to the police and tell them

Does he have a history of weird things like this? Can you make sure he doesn't have access to your car? I'd not say anything and put it in a random bin. Can you move out?

Either tracking or recording voices.

Tie it to a neighbours cat 🐈

Stick it in the nearest bin

Wait until the battery runs out and leave it somewhere random

To track or record conversations. Get it launched into a pond! Can you get out the house or get him out?

He must have a key or regular access to your car to keep charging it?

Police!

Do you share a car?
How long do you anticipate having to live together?
He’s tracking you and or recording you.
Depending on how I felt, I’d either let him hear what I wanted him to hear or keep the phone somewhere else. Don’t do anything that puts you at any risk in the interim. Can you tell someone close to you what’s going on? (Not in ear shot of any devices).

Where did he hide it?

Wrap it in some foil.

When you next take the car out alone maybe leave it somewhere public or as others have said, it can go for a ride in the bin lorry! It all sounds rather unnerving though OP, well done for bringing it up with the police.