My son has just purchased a new Samsung Galaxy S4 phone and it needed an update from new which appeared to be using his 1Gb of download very quickly.
He connected to my modem in the bus and over night it consumed 3.6Gb of data for the upgrade.
Made my account go over

Rang the little Asian / Indian ladies and they refreshed my account and have given me another 4Gb for 10 days

We are still trying to find out how to turn off the updates but Mr Google will help.
Regards
Brian