I don't believe there's any consistent procedure. I've stayed in ES all over the U.S. and it's pretty random.

You're right that they don't usually check room keys and whatnot, probably because it really isn't necessary at most ES's. My most common use for this brand is in suburbia where there's simply not a lot of foot traffic wandering in/out/through the hotel other than the guests and hotel employees. Someone wanting to "sneak" a breakfast would have to go to some effort to do so.

My only complaint with ES is that in some cases the breakfast area isn't set up to easily handle the entire hotel. It often makes me wonder if the building was originally designed with a typical restaurant (where guests would pay for breakfast and a small percentage of them would actually eat there) and later converted to an ES.