Not all security tags are made like the slim, rectangular kind you see on DVDs and such. Sometimes they will use stickers that have coils wound between the layers. These are usually white in color and about 1" square(more or less) in size.
Is there a metallic logo or other metal decoration on the purse which resembles a coil (like a disc shape wound around in circles)? Also have her check the articles in her purse for a metal object of similar design. If you find one, this could be inducing just enough to set off the sensor.
Other than that, look harder for a hidden security tag inside the purse. You may have overlooked it.
Good luck, Ray