Excessive Data Use on Z10

I have four Z10's in the family, all on Telus network. I just received notification that one had exceeded the monthly data . Upon investigation it showed that during last night (March 15/16), it used 1GB of data between 11:43 pm and 8:00am in amounts of 95.37MB every 57 minutes while connected to WIFI. I checked the other three and the same thing the previous night (March 14/15). Any ideas what is going on, as each phone has now incurred $35 of excess data! all the while connected to WiFi. The only common apps between all 4 are what came pre-loaded. Talked to Telus and they were no help.

Thanks