EDIT: Yes, that creates a data point that police could also search for — "give me all the phones in the vicinity that were turned off and then back on surrounding the incident." Okay, how far away? That would be a massively larger search, geographically, wouldn't it? How far before and after is your window for turning off and back on events?
That's still an additional search. It's also probably got a far worse signal:noise ratio than proximity to the incident — assuming the robbers were dumb enough to have their phones on them (and on) in the first place.