Okay, I am not having a lot of luck in finding the specific case I stumbled on the other day - about police using a tower dump (all the metadata on a mobile phone tower) to help in a murder case.
I can only find, at the moment, a few US and Canadian articles about this type of police action. Not specifically what I was looking for, but it shows how using tower dumps (as opposed to requesting info about one or two specific mobile numbers) is sometimes used.
One article:
How “cell tower dumps” caught the High Country Bandits—and why it matters
Fishing expeditions can pay dividends—but do they need a warrant?
How “cell tower dumps” caught the High Country Bandits—and why it matters
Another article (there are quite a few out there):
“Tower dump” production orders occur when police organizations request the records pertaining to every user whose cellular phone connects to a certain tower or towers. Each tower serves a specified geographical area, and each time a phone connects to that tower, a record is produced identifying the phone that connected, which can then be linked back to the account and account holder. In effect, these records can be used, through tower dump production orders, to identify all users in the geographic area serviced by any particular tower.
In
R v. Rogers Communications, the Peel Regional Police obtained a production order against Telus and Rogers as part of the investigation of a string of jewelry store robberies. The order sought production of all of the data from more than 21 Telus towers and 16 Rogers towers.
Tower dump production orders: Restricting police access to cellular records in R v. Rogers Communications - Lexology