I will always remember talking to a female project manager who worked at GCHQ.
She said that you could be tracked when your mobile phone was switched off.
We all laughed - but later I wondered if that was in fact true?
I don't know whether you can or you can't, but the point is who knows what is possible?
I've heard stories over the years from various sources about what was possible and what they have seen done. All of this is commonplace now - but seemed like science fiction 15 years ago.