I do all my own work, and have 3 cars to look after- 38 years old, 18 years old, and 2 years old, and it's not uncommon to start one job only to find something else needs seeing to as well, especially with an older car . Let's say someone takes their car to the garage after it's brake-pad wear warning light comes on.Naturally they would assume that it just needs a new set of pads, but.........
The mechanic gets the wheel off, and notices the discs are badly worn, he also notices that one of the fluid pipes is a bit bulgy, he may notice a bit of play in the wheelbearing: these are the sort of things that can occur and you'd expect any consciencous mechanic to phone and tell you- you can, of course, say no and get a second opinion!!
And anyone who restores classic cars will know the situation where you do a bit of exploratory rust prodding with a screwdriver, and before you know it you're into a full blown body and chassis restoration......so don't neccesarily assume they are trying to rip you off- some will be, of course, but most won't be and will have your safety in mind.