Although the other moms' responses are good I'm wondering if this ISN"T a medical issue. At that age although they may seem reliable and totally trained they have little minds that aren't always realizing that, "hey I have to go pee or poo and better stop what I'm doing to go to the potty." Part of our job while training (and it really doesn't stop til they are like 8 years old - my one child had occasional bed-wetting issues into first or second grade) is watching for signs that they have to go and ask them or tell them it's time to go to the potty. Esp at functions like parties, bbqs, Chuckee Cheese, etc. - they don't want to miss anything and would rather hold it or wait and then it comes out coz they can't hold it.
Make a practice of watching for the signs...a strained look on their face, a clutch to the pants, a stare off into space, etc. Also, be sure to have her go before you leave the house, restaurant, park (where ever you are). Ask if she has to go maybe 15 mins after eating and drinking, etc.
The other thing you want to do is not make a big deal of the accidental pee. Shrug it off and say, accidents happen, it's okay. Clean her up and move on. If you make a big deal out of it, that puts more stress on her and she'll remember that causing more accidents.