Very curious indeed.
It is beginning to appear that the person checking the truck is at the very least unfamiliar with master cylinders that are kept full from a remote reservoir.
I wonder if the brake fluid level indicator lit up after this person removed the master cylinder hose adapter at an earlier time believing that was how to check the brake fluid level.
We had a guy in our shop years ago who was under the impression that the brake fluid hose from the dash mounted reservoir was nothing but a "breather hose" until I called his bluff on whether or not he had checked the brake fluid level when he allegedly did a minor PM inspection.
The ensuing "confrontation" with him revealed his lack of experience despite his boasting of "years in the field". Maybe a corn field.