Is it possible for Google to detect app updates that have not been delivered through Play Store?
I used to play Ingress before, and I was able to update to newer versions using APKs uploaded by other players. I don't know if Google's verification process has changed, but as long as it's an official APK (i.e. not modified), it should be fine. 

Also, according to this Google+ thread, the verification process uses Google Play Services.