We are seeing approximately linear growth in our bill due to "GCP Storage egress between NA and EU" costs. As far as I can tell we have neither any storage buckets, nor instances in NA. Looking at the storage.googleapis.com/network/sent_bytes_count metric, it appears the egress might be coinciding with deployment of the App Engine app (it is a static site that is redeployed every 5-10 minutes).
How can I find out what data is being transferred from NA and how to stop this, to avoid the charges?
You can activate the Cloud Storage data access logs. It's deactivated by default because the volume of logs can be huge.
Anyway, for you case, you can activate them for your investigation, and then deactivate them.
You can also have a look on your App Engine deployment region. It's maybe the root cause.
I'm also noticing some unexpected GCP Storage egress between NA and EU costs. I'm running an App Engine app in the EU region. My theory is that this is due to container images being downloaded from gcr.io (NOT eu.gcr.io) as part of the process of deploying an App Engine version. (It says here that gcr.io is currently in the US.) I find some evidence of this in the Cloud Build history: there, I see e.g. Pulling image: gcr.io/gae-runtimes/crane:current. If I browse to gcr.io/gae-runtimes/crane, I see that its "Virtual size" is 7.66MB, so, since I've done 37 deploys by now and my bill mentions 1.58GB of egress, by itself it does not explain the figure completely, but presumably other, bigger images are being downloaded as well. (I see in the Build History things like Already have image (with digest): gcr.io/cloud-builders/gcs-fetcher, but perhaps these are charged anyway?)
