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Here's what happened:

  1. Desktop working perfectly (playing SuperTuxKart on Linux Mint).

  2. Power down (computer left unattended, no one home).

  3. After about 3.5 hours - power up.

  4. 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).

  5. I set the correct date-time and power down.

  6. Power up and enter BIOS setup (supervisor password vanished) - set everything up properly - Save and Exit.

  7. Test computer for about 5 minutes and power down.

  8. 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:

  1. CMOS battery died.

  2. 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)?)

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Commenting since all I have is personal experience to base this, but it sounds to me like you have an almost dead battery. Replace it anyway. – Bad Neighbor Jan 29 '14 at 13:01
Only you can determine if somebody had physical access to your computer – Ramhound Jan 29 '14 at 13:01
I have had many servers display the same symptoms over the years and it's just a sign that your battery is on the way out. Temperature plays a factor on how often it occurs but it is pretty much random. – Tog Jan 30 '14 at 8:41

It's probably just the CMOS battery. These batteries can take time to finally expire. It can takes weeks of intermittent behaviour before an old battery finally cannot provide enough voltage to maintain the BIOS settings.

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Compare your CMOS battery to one in your television remote control. You won't find it has died all of a sudden. You will get intermittent use out of it maybe for another week as the last bit of life is drained from the batteries.

Your CMOS battery works the same way.

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CMOS failures always corrupt CMOS checksum. In such cases, BIOS shows following error message and asks for new time/date. But you didn't mention any error messages, therefore someone accessed your computer when you was out.

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