Perhaps they have a family Life 360 account and Bea could see the shared location. Just speculation.
What I find incredible, apart from the fact they were scheming together and lying to Bee, is that they even parked over the road from the police station when they had no intention of going inside!
Why did they do this? Maybe the answer is as simple as taking a photo to send Bee, but what on earth did it achieve? They still sent her to the grandparents telling her she was lying.
Was it supposed to show Bee was lying if she told anyone Sarah had threatened to kill her if she went to police? If Social Services investigated they could show a picture on a phone and hope they were believed with no police incident report?
It does show however that these two were already intent on perverting the course of justice, using their phone locations or CCTV to set up a false trail for a purpose we don't know. They are peas in a pod, a complete match for each other, no remorse, no regrets, I wouldn't even call it a sacrifice because there didn't appear to be a great loss or emotional upheaval involved. I can't get my head around people like this, so disconnected.