A couple ways. She can dig around in people's memories, although if there is a strong magic hiding that memory then she may have trouble, especially in her current state.

If Peter can remember where 'something weird' happened, or the general vicinity of the event, she could be able to sense and view the memory of that place. Something like that would have accrued some dark energy, especially so recently.

Otherwise, it can just be her stalking around the city hoping her demon senses will go off around the right person, and so it could be happenstance that they run into each other and a connection could eventually be made that way, if not immediately.