I upgraded my Nexus 5 to the latest Android version. 6.0 aka Marshmallow, and didn’t really think much of it. Then, on Monday, I needed to run a test on some software that required the Wi-Fi to be turned off. I noticed at the time that the GMail app was having some problems; I received regular crash notifications from it (while it was running in the background).
Then tonight I checked in on my test and was surprised to see an Android mobile data warning in the notifications area. Tapping through, I saw that in the last few days I have consumed ~5GB of mobile data. On a phone that has been sitting on my desk without me touching it. The culprit? You guessed it, the GMail app was responsible for almost the entire 5GB. In less than five days.
I also received an OS update this evening, and since then the GMail app seems happier. I am back on Wi-Fi now though and will continue to monitor it. Had I been on a 3GB plan, I would have been more than a little annoyed (and had to pay for the extra data). As it happens, this phone is on a 20GB shared plan, so I have some headroom.