To be clear, a $2M home in Millbrae is no mansion. Homes in Millbrae cost an average of $964/ft². The national average is $123/ft². It's equivalent to about a $250K home in an average town.
Not a mansion, but it's still notable that an officer could accumulate a 20% downpayment with post-tax earnings. That's $400,000, which would require banking $40,000 of after-tax earnings per year for 10 years. That's perhaps $60,000 in pre-tax earnings.
Or he formerly owned a home which cost a lot less and could easily sell for $500k profit.
Also the annual property taxes on that place would be substantial