Think about who would install a tracker;
A soon-to-be ex-husband. They watch the telly, and know from old episodes of Magnum PI that the best place for any tracker is under the rear bumper or up inside a wheel arch. Simple to find, and they have very short range because they're wireless only, not GPS.
A car-hire company. They do this all the time. Usually behind the dashboard, because they want it to be hard(ish) to get to, but pretty simple for a mechanic to install quickly. That way, they can access power, and also have a cut-off switch so the car can be disabled remotely. These need to be removed carefully, so that the car isn't disabled. Simpler, battery powered trackers can be on the dashboard or parcel shelf; they don't have much power, and GPS trackers need to be able to see the sky (under-dash versions use an aerial).
A government agency. Forgedabodit... you'll never find a tracker installed by a government agency. But as you'll never know about it or ever see one, don't worry.
In my opinion, you're probably looking at an ex-rental car, and the owner wants to make sure that there are no 'extras' left behind. She could just call the rental company and ask them if there is anything.
For your specific case, do the repairs, and report on any odd items you see during your repairs that aren't shown on the workshop manual, or look like after-market. Don't remove anything (in case there is a disabling relay in there). Don't strip down areas that take a lot of work that aren't involved in repairs, and report that you couldn't find anything.
Unfortunately, you can never prove a negative (so you can't say 'there are definitely no trackers in this car') - but you can say that your opinion is, that in the areas you worked on, there are no obvious tracking devices.