Poor Zoey! Is she itchy? Vet Medicine says dogs do not react to poison ivy:
Do Dogs and Cats Get A Poison Ivy Rash?
But maybe Yorkies are different? I can confirm what the article says about dogs carrying the irritating oils in their coats and transferring them to humans. That happened to my brother who ended up in the hospital after our dog picked it up in the woods.
I don't have any other ideas what might have caused the patches on Zoey.