The answer is:
Statistics - the noble art.
Oh, and famous London cctv.
And as you wrote - Petra's "Eastern European accent" which should attract more witnesses as a distinctive feature.
PS
Please, let's leave it at this![]()
You've raised the issue so I am addressing in return - it's simply not a given that CCTV and witnesses are always privy to a disappearance. Unless Petra was walking down the road, talking to herself and waving at every camera she passed, she could have slipped under the radar quite easily. Take Andrew Gosden for example, he was last seen in King's Cross and it's one of the busiest places in London!