Thanks for asking. It means that the Covenant Eyes (CE) servers are not receiving any signals from the Username connected to that Accountability Report.
When I sign in with my Username, the CE app/program will send a check-in signal to the servers. If the servers don't get a signal from my Username during the reporting period, then the report for that period will say that Device Activity is Missing for my Username.
What seems to happen quite often is that Person A helps Person B install the app, and Person A stays signed in on Person B's device. So Person B's Username is not sending a signal, and gets a Device Activity Missing report.
If this reply doesn't make sense, please say so or call our Customer Service Team at 989-720-8000.