Every woman has different body chemistry.
Do not for one minute think white stains in undies means sexual activity.
It could be the onset of a yeast infection that many women get for almost no reason at all.
You can get one from eating too much starch to wearing clothing from the store before washing it first,
to simply getting one just before your period comes on.
Most women`s natural moisture or discharge is clear and makes panties moist.
Some women are more dehydrated and their discharge is thicker and appears white.
It may not be anything at all.
Try an open conversation with the wearer of the panties.
Maybe she`s too embaressed to even talk about it unless you bring it up.