I'm not aware of sensors that read octane, but there must be SOMETHING that the computer reads to know the difference in E10 and E85 to automatically adjust the tune. Supposedly "perfect stoich" of ethanol is not 14.7:1 like regular gas so the computer can't read JUST the O2 sensors, can it?

And not all cars have knock sensors...

I'd guess it was just a tank of bad gas. Want to test your theory? Get premium gas from another station... Chances of getting 2 tanks of bad gas from 2 different stores "back to back" is very slim.