I think it's a last minute thing for my DD, too, or anticipation. We were on a nature hike and I pulled DD over to a tree and she peed as soon as the air hit her bare butt...all over her skirt and panties...and my shoes...
My DD is nearly four and doing the same thing with mostly tiny accidents. Dealing with it has been very frustrating. When she has an accident, I tell her, "If you had gone when you first thought you needed to go, you'd be done. But now that we have to wash your leg, change your pants and socks, etc. it's going to take us a long time. Not really worth it to wait, is it? Even a little bit needs to be changed because pee smells."
DH thinks it's an attention getter b/c I work from home but I may show him this thread to say "It's not just us!" I did point out to him that our friend's son (SAHD, no side job) had problems with pooping accidents when he was 4 so be glad it's just pee.
I've gone back to reminding her to try to pee before/after meals, before we go anywhere, etc. I told SD to take DD to the bathroom before shopping at every store when they were running errands yesterday. Even if she says no, take her anyway. When DD gets annoyed with this, I tell her that she doesn't always go to the potty when she needs to, so until she is always dry, I will be reminding her.
I know she can stay dry. She stays dry better when we are out and about. It's like she just isn't embarrassed enough to not pee on my carpet but she's motivated not to have an accident in front of her playmates.