[farseer]

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.