In one office that I was helping out in, there were several APC UPS units - none were designed for PFC supplies. They too had a power outage due to a storm - after that, they had two computers that kept going into hibernation at random times. APC walked me thru some tests and determined that the UPS units were fine (I have my doubts), but they suggested not putting them on these types of PSUs.

That office has not purchased the more expensive UPS & I ended up removing the UPS units from those two computers. Neither computer had this hibernation problem before that one storm/outage. If it were up to me, I would replace the PSU in one of them just to see what that does.

All of the computers in that office use XP and the APC Powerchute app. If you will notice, I only have W7 inside a VM. I don't use W7 for day to day computing - so I don't know what kind of detail to expect in the W7 logs for this type of APC UPS. The APC Powerchute app sometimes mentioned noise on the line before putting these two computers into hibernation. I don't know if the Powerchute app traps more events than W7 does.

You mentioned coming back from getting coffee and finding the computer going into hibernation. Did you nuke your coffee in the microwave? Or do anything that could disturb the power?

Two free things that you can try:
1) Sit back and watch the computer for the length of time that you were away getting coffee - see if the computer hibernates when nothing else changes.
2) Install Powerchute and see if it gives more details.