What a strange twist. I imagine it was pretty simple to find her. If she came in to Heathrow then she probably took the shiney new camera'd-up Elizabeth Line into London.
So they would have know where she got off. Even if she somehow then managed to slip all the CCTV, unless she had a stack of £££ she would have to start using card or getting money out.
Chances are she used her card to book a room, even if she didn't. Check the local hotels, Uber, Black Taxis, buses (cashless and contactless). She'd have been leaving a fair footprint unless she immediately holed up at a local friend's house, and they'd have been looked into.
Can't imagine why she'd pick the UK of all places. Ok you got the language, but it's a small island, big ties with the US, extradition treaty, reasonably competent police and you definitely need a passport to get back out the country unless you really know people. The only conceivable place to slip into without one is Ireland and then you still have the same problem.