Wifi per app monitoring on iPad
s3-us-west-2-w.amazonaws.com

AWS in this domain name stands for Amazon Web Services, while S3  stands for:


  Amazon S3 (Simple Storage Service) provides a fully redundant data storage infrastructure for storing and retrieving any amount of data, at any time, from anywhere on the Web.


US and West probably refer to the location of those servers. (I use Amazon's Glacier to backup some of my data to the cloud so I recognize this pattern)

This means your iPad is uploading data to Amazon's cloud servers, but it doesn't necessarily mean it's Amazon's apps, since many developers utilize AWS within their own apps for tracking, monitoring, hosting, and storage. 

Proposed solutions:

I suggest either/both of the following, none of which are "perfect" solutions:


You're saying that no matter what you do, the data is still uploaded. This means that this app has "Background App Refresh" enabled. Go to Settings, General, Background App Refresh, and make a list of "prime suspects"1, disable background app refresh for these apps one-by-one and see if the uploading stops after you disable the background uploading for this app. (The app would still be able to upload if you use it)
(This won't work if your iPad is WiFi only) iDevices actually provide you with stats on cellular data usage by app. Go to Settings, Cellular, and scroll down until you see a heading: "Use Cellular Data For". Now, take a couple of screenshots at different times, and see if the data usage increases for an app that you haven't used since you've last seen the uploading process.


1 Would be really helpful if you edit your question with this list as well.