I almost got mad for a few weeks trying to check how a web page was updating itself. No network traffic, nothing found with Fiddler, service workers were not used, no background sync intercepted..
On the left you see the stock price updating in real time, on the bottom no network traffic was intercepted, and on the right neither any background service
The issue was related to the antivirus:
As soon as I disabled the antivirus (Kaspersky) the traffic from chrome's Service Workers became visible again both with Fiddler
And Chrome