Advice for tracking down a listening device?

Start resetting devices first. Like phone and laptop. Then change password to social messaging apps and email. Also change WiFi SSID + password. Be patient and observe a bit. If still an issue then engage police again.

I would suspect her phone first before looking for a physical device. An android phone is particularly vulnerable to being backdoored by cyber criminals that you can hire. Also any other computer or laptop in the house can similarly be compromised.

After that, the wifi thing is easy to check as you mentioned, but a dedicated listening device can also have a build in LTE radio with a SIM card. There is no upper limit on sophistication, even a completely passive device without external power is possible if the hostile actor has the money.[1]

[1] https://en.wikipedia.org/wiki/The_Thing_(listening_device) Stuff like this was possible in the 1940's imagine what is possible now.

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.

I would suggest to enable logging on her router (if possible) and to check the MAC address of all devices that connect to her home Wi-Fi.

On smartphones, tablets, and computers, it should be easy to find in the settings (but check if they've set up a rotating private Wi-Fi address; if so, disable it).

It might be more difficult with other devices: for televisions or other appliances, you might need to unplug them and check the router logs to see if a specific MAC address disconnects at that precise moment.

Ultimately, if you have a MAC address that stays connected all the time or even just occasionally and doesn't match any known device, you have a good candidate for the listening device.

Some routers can block internet access for new/specific devices. You could try to find the correct device by blocking them all and unblock them after finding and checking them. One possibility is that the stalker uses an old android. That would harder to detect, since its possible that the device is not connected via wifi.

Cryptomuseum.com has information on listening devices and countermeasures that some kind poster on HackerNews linked a long time ago. https://cryptomuseum.com/df/tscm.htm

For the WiFi stuff you're asking - just change the SSID and password. The listening device won't have any way of knowing the new password. Consumer devices could easily spoof MAC's, but generally don't.

However, malware on laptops/phones also needs to be ruled out. The most thorough and least technical way to do this would be to sell them and use the money to buy equivalent used devices on the local market. Transfer documents, but don't reinstall everything from a complete backup (re-install apps manually). You'd also potentially want to throw out / sell / repurchase all the USB cables in case one of them is actually a tiny computer in disguise which can be used to hack laptops and make them record audio: https://shop.hak5.org/products/omg-cable

More generally about finding listening devices:

The device needs power and some way to transmit information (signal).

Power would be a difficult path to locating the device. It could be inside another electronic, like a clock, TV, or smoke detector. It may also have a battery to continue working for some time after it's unplugged. It could theoretically be in a wall / junction box "hardwired" into the home power.

It probably must either be connected to the local (home) network, broadcast electromagnetic signals (cellular, or generic short/medium-range RF), or the perpetrator must drive by periodically, presenting or accepting connections to a unique SSID.

If the listening device is using WiFi, it's relatively easy to buy a new router (to ensure there is no listening device inside the router itself) and set it up with a new SSID+Password, and configure it to only allow devices on a "whitelist" of specific MAC addresses. However, this won't help if it's connected to a neighbor's WiFi or using something like Amazon Sidewalk.

If the device might be using a Cellular modem or analog RF: SDR's (Software Defined Radios) could be used to scan for signals between 800MHz to 6GHz to triangulate the sources of any wireless signals (this range would cover analog RF, cellular, and WiFi). As long as the device is regularly/frequently emitting signals, the homeowner could turn off each device (Phones, laptops, TV's, smart thermostats/appliances, router, etc) that emits signals between 800MHz - 6GHz. Some items may surprise you - portable HEPA air filters or body weight scales or smoke detectors might all have benign wireless emissions - for these, it would be good to identify their model # and check if they are advertised as having "smart" features. It's also possible that one of these devices might have been opened and a recording device placed inside, in which case you can't rule them out completely except by selling and replacing them all.

However, there is a small chance (given that "the stalker is not technical at all") that the device could lie completely dormant until it receives a "wake-up" signal on a particular RF frequency (the stalker would drive by and emit the "wake-up" signal and then the device would respond by sending the recorded audio). If that's the case, you'd probably need multiple SDR's monitoring 24/7, and calculating the approximate triangulated position of each source. Rule out all the sources of frequent RF emission, and then have the system notify whenever a new signal is found near the home, and try to locate it and rule it out.

It's a pretty tough problem.

But most likely, it's using WiFi, or maybe a cellular modem. The homeowner's WiFi/network is easy to lock down. However, if it's using a neighbors WiFi or a cellular modem, you'll likely need to use an SDR. If it's super advanced, you'll need to use a constellation of multiple SDR's with 24/7 monitoring to locate it.

The most advanced but still technically feasible technique to find a listening bug would be to use a "Non-Linear Junction Detector". They start at about $15,000 and will find pretty much any semiconductor in the building. But the device may be hidden inside of an actual appliance (TV/etc) or dressed up as a "normal" electronic device (like a bedside clock or smoke detector). So this would only be useful for finding some small electronic device that's not, itself, already hidden in some electronic appliance.