Here's what happened:
Desktop working perfectly (playing SuperTuxKart on Linux Mint).
Power down (computer left unattended, no one home).
After about 3.5 hours - power up.
Linux Mint starts up almost perfectly with the exception of system date and time (date-time as reset to motherboard default (1st Jan 2008/9 - 00:00:00 UTC).
I set the correct date-time and power down.
Power up and enter BIOS setup (supervisor password vanished) - set everything up properly - Save and Exit.
Test computer for about 5 minutes and power down.
Next day (at least 15 hours later relative to step 7) power up - CMOS working just fine - password did not vanish and date-time is correct.
Based on my experience this could have happened if:
CMOS battery died.
Someone tampered with my computer (physical access).
If it was the CMOS battery - it should have kept losing the password and the date-time (repeated CMOS failure). But this did not happen.
My question is - Could the CMOS fail just once, and NOT because of the battery? (OR did someone tamper with my computer (physical access)?)