OK, since there are no fleas and you are using Frontline regularly on both the dog and the cat, and since the area does not appear to be itchy, it probably is not an allergic situation. Most of the allergic issues (Flea allergy, inhaled allergy, food allergy) all tend to be itchy and often are associated with skin lesions.
Areas of hair loss that are not itchy and have no skin lesions narrow the field quite a bit. Here are the things I might consider:
1) Demodectic mange. These mites can be demonstrated with skin scrapes. They are treatable with Lyme sulfur dips. These are not very common.
2) Ringworm can sometimes manifest as hair loss without itchiness, though there are usually some skin lesions. This is ruled out by doing skin and hair cytology and performing a fungal culture.
3) This could also be an area that just got rubbed off by something as benign as excessive grooming or a mild abrasion that will cause no issues at all.
4) Occasionally when we have hormonal abnormalities, we can see hair loss that is symmetrical on both sides and non-itchy. Things like Diabetes, thyroid abnormalities, etc. can result in lesions like this. This is much more common in dogs, though, than cats.
Here are my suggestions. I would first suggest a little bit of benign neglect. Since it is a singular lesion and does not seem to bother the cat at all, I'd wait two or three weeks and see what happens. If the skin remains normal and the area of hair loss resolves, then you don't have to do anything.
If any redness, crusts, scabs, itchiness or irritation develop, then I would not wait longer. If the lesion fails to resolve within two to four weeks, or if any of the above signs develop, then I would take her in and have your vet evaluate the area. It would be a good idea to do skin and hair cytologies under the microscope looking for mites and evidence of ringworm. I would also think a fungal culture may be wise. If these are OK, I would do some bloodwork to rule out any hormaonal abnormalities.
The good news is that this does not sound to be too serious. Hopefully it will resolve on its own. Hope this information has been helpful. Good luck with this.