Cell phone companies record the tower that handles each call, and then
keep that info for at least a short time.
That came out in the recent arson case in Maryland. The suspect
claimed that he was at home. However, he had a cellphone call during
his alibi time. His provider's records showed that the cell tower
handling his call wasn't near his house.
This had to have been a routine log since the suspect wasn't yet a
suspect at the time of the call.
(Putting my tinfoil hat on:) I have a theory that the location of
every cellphone that is turned on is being routinely recorded, say
every minute, and saved for later data mining. I have no direct
evidence. However, this is technically feasible, and is a logical (to
me) extension of what we know was done decades ago.
