Hi A.,
I used to be a preschool teacher and encountered a similar situation, but in this case it was a boy. And it was not just pee! It was, however, permanently remedied by having him take charge of his accidents. To own them, if you will. It sounds like this has become a power struggle between the two of you. I suspect that if you diffuse the struggle by empowering her to take responsibility for her accidents, they may cease.
Try approaching the situation using the 3 R's of discipline: Respectful, Reasonable and Related. Here's what I did and it worked!
First, set the ground rules so there are clear expectations (and get ready for her to test them). Explain that you and she are going to work together on this. That you will ask her if she needs to go potty from time to time and see if she will agree to try going. But don't force her to take potty breaks, let her decide if she needs to go when you ask because you want her to feel in charge.
If you are not already, try asking her if she has to go at strategic times when she's less likely to be totally engrossed in an activity - like before/after meals, before/after naps, before/after playing outside, etc. This could really help.
Also explain very kindly that if she does have an accident, that she will be completely in charge of cleaning it up- changing her clothes, wiping the floor, putting wet clothes in the laundry basket, etc. Let her know that if she needs your help she is to ask and you'll help her, but that she will be in charge.
The next time she has an accident, act very matter-of-fact about it. Respect and understanding are key to diffusing the power struggle. Be careful to not show disgust or disapproval or make her feel ashamed about the accident (which could leave long-term emotional scars). Empower her by guiding her through the process of cleaning up, but let her do most of the work (just like you agreed).
Be reasonable- she doesn't have to clean the whole house, but eventually, she will learn that if she had gone on the potty she could still be playing. And that going in her pants will no longer get from you (or anyone else in the family) any negative attention. Make sure Dad is on board with this approach and that you are both consistent.
Lastly, consider that there may be an underlying motivation for these accidents. The little boy I worked with had an older brother with special needs who demanded the lion's share of his mother's attention. When I started at the school, I was told by the other teachers that when this little boy had accidents, he was to be sent to the babies' room to get put on the changing table and cleaned up! After ruling out a medical explanation for the accidents, I suspected that he really wanted the personalized attention that this gave him and that it was comforting to him to feel like he was one of the babies. A few weeks after finding more positive ways that he could get attention, working with him to be mindful about his need to go to the bathroom, and respectfully empowering him to clean up his own messes, the accidents stopped completely.
I hope this helps you, too. Good luck!