Have the customer pop the hood (even if they only want you to check the tires' pressure or wipe the bugs off the windshield) and go over anything that looks/smells/sounds a bit "hinky" while they're standing there.
Have them sign-off on the original repair. (Yeah, like that'll happen...)
Sounds like you almost have to have in-house cameras and employee-carried cameras to document your truthfullness.
Then, again, some customers would demand a complete "repaint" if they found a scratch on the car that was there before they "gave" it to you to do what you needed to do.
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Rule #1.) The customer is always right.
Rule #2.) See Rule #1.
Rule #3.) If the customer is still an @55
AFTER you've shown them Rule #1 and Rule #2, fix their problem, then kick their whiney-little @55 out of your shop and forbid them to ever return. Most of their acquaintances ALREADY KNOW THAT THEY ARE WHINEY-@55ES, so what they say will probably never affect your business or reputation.
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Life's too short to worry about idiots like that...as a business-owner, you still get to pick who you like to play with.