The two common reasons - if nothing else has been touched - are damp filament, some filaments change surface quality when damp and can begin to slip, and filament diameter: if you change to a filament with a slightly smaller diameter - measured, don't trust the adverts; then it too can slip.

Dirt or dust on the spool can get into the works and cause problems ...

and if there is excessive build up of dust around the Bondtech gears they may clog and loose grip.

Does it not have a way to know that it isn't feeding filament?

That depends where the problem is - often not.  The older style of sensor was more capable at this but failed with certain colours.

Cheerio,

I learned how to fix that, and printed about maybe another hundred hours and midway through a long print (again) it stopped feeding filament, but continued to pretend to print, just no filament came out.

I was able to heat up the hot end to about 10c over the normal petg temps, and pull the filament out. it had a bit of extra filament on the end.

With subsequent reheating, I can't get any filament to load, just the grinding noise. I'll be taking it apart again to clean out the hot end.

Two Questions regarding this:

1.) Why did it keep printing even though no filament was being run through the nozzle? Does it not have a way to know that it isn't feeding filament?

2,) Why would I get another clogged hot end, or whatever it is again so soon? 

This is why I gave away my Ender 3 Pro and spent the money and time to purchase and build the kit. I was expecting rock solid reliability.

Any education, tips, guidance, etc will be appreciated, waiting to clean out the hotend until I read something helpful. Maybe I need to do something else when it clean the hot end so I can get several hundred more hours before this happens again.

I3s+
Prusament PETG
Factory nozzle that came with the printer (.40)
Sliced to .15 for better detail.
OctoPrint
Printer is in a closed Lack Enclosure, no extra ventilation. 
20% infill.

Thanks smart folks.

-= bruce