The fact that it wasn't really an emergency and is just a bad habit she has (want to pee NOW, pee right here wherever I am) I think does make her toilet habits pretty dirty, yes. Peeing in a field I don't think is so bad. But I'm imagining a 5 year old peeing down a drain cover at the side of the road, which is what you say she normally does. That's really vile.
A lot of kids don't want to go wee when there's something more interesting to do. Hence always seeing kids doing the hoppity dance, or clutching at their groins, when they've left it way too long. My son used to do an amazing John Cleese type walk when he was busting while STILL denying that he needed a wee. Sometimes you just take them and tell them to go instead of letting them say no. It's usually pretty obvious when they need it, or when it's been a few hours since last time. In these refusal to wee when I need to situations, I do think a few wet pants is a good way to teach them that yes, when they need to go, they need to go, and if they refuse to use loos when they need to, this is what happens.
And before I am accused of being prissy about peeing, I pee all over the place when I'm out and about in the countryside, but always find somewhere private to hide first. If it's not ok for an adult to piss in the car park, I don't think it's ok for a 5 year old either, really, she isn't a baby who has no bladder control. Only a couple of months ago DP and I were both absolutely busting in the car, spotted a little country park sign, zoomed off the road, into car park, (evening so it was completely empty) and both ran off into the woods. Even though no-one was there and we were desperate we still wouldn't have just pissed in the car park.
Unless she does have an issue with bladder control generally. In which case, take her to the doctor.