I have the same problem.... Pixie is pad trained but only in our house. I think she's just learned to go in those couple spots and doesn't necessarily understand that she's supposed to go on the pad. I could lock her in the tiniest bathroom with a pad at a friend's house and she'll still find the one small area that isn't covered by it to go!
Fortunately, I know her potty dance. She runs around sniffing... almost in a frantic manner... She only does that when she needs to go! (She'll do it at home as well as other places). It's really subtle, my husband still can't see it.... I've just learned to watch for it and the second I can tell she needs to go, I take her outside. Then I get to stand out there for 30-45 minutes while she just stares at me like "What the heck are we doing out here?" She's not used to having to potty outside
