I got this tip from a professional paint sprayer.

Close your eyes and run your hands all over the bodywork - anyone looking will think you are a total fruitloop but I find un-noted damage EVERY time - no fail.

Not sure about other outstations, but at BRU all rental companies ( 5 or 6 I think) have a phone to the front desk when out of hours. The damage is taken as read verbally and noted.

At a rough guess the technique I use has probably saved me in the region of 300 euros a month (given that some dents are very small and not noticed anyway).

Also the choice of rental company dictates what is a 'dent' - some are far more lenient.