You can't track the watch. What you can do is to see if they log any new activities. Will probably not happen if they know what they are doing.
They can see the information you've entered into the watch. There are some physical data. You can see that data in Garmin Connect in the watch settings.
They can also access the FIT files on the watch and see what you've been doing and where if you use GPS during your activities.
The watch can't be used to access anything in your Garmin Connect account.
