Usually participles in the introduction part describe the subject in the main clause, but considering the translation and context, "unknown" seems to describe the whole main clause meaning Joanne's same activity. So is this an exceptional case or a common one?

p3)So not only were her fellow drivers angry at her
, it was actually taking her even longer to get home after work. Kate’s children were almost ready for bed by the time she walked in the door. Feeling bad for her family, Kate gave up on her plan of not blocking the intersections and went back to driving like everyone else.
But unknown to Kate, her neighbor Joanne, had come up with the same idea
a couple of weeks earlier. Driving to and from her office, she had been waiting at the intersections to let other cars pass. But her experience was just as bad as Kate’s, and before long she had also given up.