Much of the UK MOT test is subjective - ie some things are not actually measured with instruments, but it is the opinion of the experienced tester that there is some wear, free play etc that is either unacceptable and fails or is acceptable at the time of the test but may require attention sooner rather than later. Ther 'advisory' alerts the owner to the neeed to 'keep an eye' on the situation, but is also used by some testers to 'cover their backs'
MOT tests are overseen by VOSA who sometimes carry out checks to ensure the efficiency of testers , or mediate in disputes about whether a car should have passed or failed the test. Advisories are sometimes used by the tester to prove to VOSA that they did indeed spot there was some wear, or other anomaly but decided it was acceptable at the time of testing. . I did once hear of an advisory being given to the effect that at the time of the test the car was fitted with wheels and tyres that, although legal for a pass , did not appear be its usual set.
Brake discs do have a minimum recommended thickness that can be measured with calipers. But worn discs also typically develop a lip of unworn metal around their outer perimeter where the brake pads dont quite reach. A tester might just conclude from the presence of a lip that there has been some disc wear without actually measuring it. It may still be well within acceptable tolerance. Discs are sometimes failed/advisories because they are excessively grooved or rusty .(some grooving etc is considered acceptable. ) This could be mis described. This type of disc damage/ rusting is quite common on cars that are driven infrequently and on short journeys.
Incidentally 'sloppy' mechanics sometimes also estimate brake pad wear without bothering to use instruments. And this can be deceptive as the metal backing plates of pads are sometimes dipped in black paint which depending on the pad maker may extend partially up the lining material. Mechanics will sometimes 'estimate' the remaining thickness of pad lining material to this paint line rather than its actual thickness.
But ultimately its best not take risks with brakes, or speculate on whether they really need doing or 'how long have I got' . I'd get them checked independently.
