Probably the police wants to gather some evidence. The easiest way to block a wifi tap is to change the wifi password... or wifi name. Both things will be noticed by the eavesdropper because they will block the device *but* it is an easy way to block it asap.

What I would do:

1 - Get an old android phone. 2 - Configure the phone to act as an access point with the same wifi name and password. 3 - Change the wifi password or name in the router. 4 - Change wifi passwords in laptop, etc... (I know, it is a pain). 4 - Activate the phone access point. 5 - * IF * the listening device connects to the phone AP wifi, you know there's a wifi tap. You can connect the phone to your home wifi later... and it will look transparent to the stalker.

Best of luck with this situation.