iPhone using data in flight mode?!

we have just started an overseas holiday and before our flight took off I did 4 things to my girlfriend's iPhone 5S:

1. Turned off roaming data

2. Turned off cellular data

3. Turned on flight mode.

4. Reset cellular usage statistics.

I Double checked the counter and it had definitely reset to 0.

But I just checked the counter and it had 732kb?! (Under the data counter not roaming data counter)

is this just a bug with iOS?

she turned off flight mode briefly to get SMS but cellular data was still completely off. So I don't see how it could have downloaded anything

Grateful for people's thoughts. Thanks.