Even though there are no users with ActionTracing enabled via Resource Properties, the app-trace.xml files are being populated with every user action.
In the trace log you can see the following type of info:
<INFO date="2018-08-08 06:54:42,860" user="clarity:XOGUSER" session="24932016__61618432-D803-43DE-B12C-AB6E677D31C0" action="homeActionId" thread="http-nio-8080-exec-2" category="trace.server.threshold">
This shows the category being invoked is "trace.server.threshold"
Release: 15.x, 16.x
Component: STUDIO