Thanks Rafeeq - On the client, we went into the UI and changed the Virus and Spyware settings =>Global Settings=> and unchecked Enable Insight for:
The manual scans consistently reported around 174,000 files scanned.
We went back in and check Enable Insight for: Symantec Trusted. The manual scans reported 81,000 files consistently.
However, that still doesn't explain why, with Enable Insight for: Symantec Trusted, the first full scan after AV updates reports 174,000 files scanned and then immediately running a full scan afterwards reports 81,000 files scanned. It appears that the AV updates causes everything to be rescanned for the first scan after the update.
This may be normal behaviour. We don't know as this is our first testing with SEP 12.1
I'm going to open an incident with Support and see what they say.
For now, I'll mark your answer as the solution/potential workaround.
FYI, for custom scans there is an option to uncheck Insight checking as well without changing the global settings on the client.