Looked at making a carrier change and had my Note 3 unlocked by AT&T in September. Normal usage for a single month is under 2GB for a family plan shared with the wife. We've not gone over the 2GB plan in over 20 months of service with AT&T. For the past two months in a row we've gone over our plans even when being out of the country with our phones not connected to the AT&T services.

Month 1: Switched to a T-Mobile SIM card prior to departing the country and was within our normal usage. Gone for 4 full days. Returned from the trip and placed my AT&T SIM card into the phone. Within 1 hour I we received a message stating our allotted 2GB of data was almost completely exhausted. Ended up going over for the month by 1MB.

Month 2: Same situation as Month 1 but out of the country for a full week. Within 1 hour received a similar message.

When I spoke with AT&T and looked online, it showed that I had used 64,000KB of data at 3PM yesterday which is impossible as my flight hadn't landed. Assumed this was looking in arrears for a 3 hour period but AT&T couldn't confirm.

How does AT&T track the usage for a period? Does it take the information directly from the phone or does it take from the system? What could have changed in the following 2 months to drive me over by so much?

When you took out your AT&T sim card, you just let it sit somewhere and not putting it in another phone right? There should be no activities on the line (voice or data or txt) on the days that you took the sim card out. Now if the data activities started after you put the SIM in...it could be you have pending messages.. maybe with pictures or video waiting for you and you don't have WiFi when you put the SIM back in? Or a software update push by AT&T?

Correct. The SIM sat outside of the phone until I returned on both trips. When traveling used the T-Mobile SIM to make calls but mostly for data.

Perhaps apps being updated automatically when AT&T SIM inserted and phone not connected to Wi-Fi?