There are different devices that can be concealed in or under the vehicle. There is an older one that had to be placed covertly, then retrieved a few days later & have the data downloaded into a computer. Then, perhaps, replaced. Some of the newer & more expensive ones give real-time locations. Some are also small enough they can be placed in suitcases, etc. They are all subject to discovery.
I had a case that proved the husband was telling the truth & simply reported that to the wife. She regained her trust in her husband. If he had learned that he was being investigated, their relationship clearly would have suffered, so you just don’t tell & hope they don’t find out.
From your post.....”so you just don’t tell & hope they don’t find out.”
Isn’t that what the cheater is hoping too!
I hope that she learned something not only about her husband but herself. Any friend that I’ve known who discovered a spouse cheating “knew” for quite a while (deep in their gut). Perhaps some people are neurotically suspicious or overly suspicious because of past experiences? Maybe that ingrain suspicion twists things in your mind. Perhaps when all is said and done, their relationship will be better because his fidelity is confirmed. I hope so!