Can someone monitor your location from another phone?

If you have a list of contacts in the ‘People’ section.. you can select each one and see if you are sharing your location with them. If you are sharing your location with them, there will be an option to ‘Stop Sharing My Location’. If you are not sharing your location, you’ll see an option to ‘Share My Location’

Yes.. IF you choose to share your location with them in the Find My app.

iCloud - Find My - Apple

You can also go to Settings > Privacy > Location Services > Share My Location and there will be a list of friends you’re sharing your location with. So you could have that switch on to share your location and not have anyone listed there.. then no, they should not be able to track your location.

If you've shared your location with them from imessage too. Maybe they shared your location from imessage and you didn't know it.

There is a setting on imessage that is 'share until stopped'.

Also, don't forget, they may not be using your phone as a tracking device, but something on your car.

No, you should never give out your passwords or credentials