GSM noise in speakers even when phone is not receiving SMS or changes transmitter

As most GSM users know, you can tell that a GSM phone is sometimes
transmitting data, if it lies next to a speaker or headphone. For
example when the phone receives an SMS, or when you travel and the
phone changes to a different transmitter station.

My question is: Why is it that GSM phones sometimes transmit data,
even when the phone does not receive an SMS or changes the transmitter
phone?

If this happens for no apparent reason, could it be interpreted as a
clear sign that the phone is being "homed" or positioned, for example
by the police?

Or are some carrier networks simply programmed to, on a regular basis,
check up with the phone if it's still turned on?
(If it's relevant, my carrier network is Vodafone of Sweden. (If you
wonder wny I don't ask them directly, well it's because their customer
support department never respond to the email that I send them, for
some funny reason))

Please email me a copy of any replies. Thank you!