After a few good days it occurred again. WhoCrashed did not find a minidump to work with. However I went through the event list again and while I still couldn't see an entry from the time of the failed awakening, I noticed the following error in the System event list:
The timestamp matches the time I sent the PC to sleep last night. Maybe something goes sideways while going to sleep? Or it could be that it froze so fast that it couldn't update some data and after the reset it still thinks it was reset last night...
The previous system shutdown at 9:48:01 PM on 2/25/2016 was unexpected.