Rarely, if you are navigating at sea or in the air or in the woods... and even on the road, it is not uncommon for my GPS device to be clearly off without justifying the conclusion that there is a fault in a satellite.
* Air and sea port operators and navigators
* Military personal running supply lines
* Military personal on foot in operations and training
* Space-X
* NASA
* River boats
* Fresh water fishermen
* Etc
Out of all the possible users who would notice a 300m deviation just based on visual reconciliation, I personally would not say it would be so rare that the USAF would not find out very quickly. Of course, this is ignoring the equipment that would likely detect the issue way before somebody in the Army started phoning the USAF.
There's not so many GPS satellites out there that you're going to be bouncing around them all the time - even if only one is affected, it would be very noticeable for extended periods of time.