> one of the reasons to keep your engine bay clean isn't for it to look pretty, but so that problems (esp. leaking fluid or oil) are more obvious

That's a very good point. But, in my case, I would counter it with the fact that the car had a brake fluid change performed and that they inspected the braking system as part of the inspection process (hence how they determined the rear rotors would not pass inspection).

An essential part of a brake system inspection is to check if there are any leaks by examining the brake fluid reservoir, master cylinder, brake lines, and calipers. I would understand if they missed a power steering fluid leak or a coolant leak since they weren't directly dealing with those systems as part of the service I paid them for, but they should have definitely spotted that leak before giving me the keys.