when ordered to reveal a facedown damage/horror card, you shuffle the ones you have and then reveal one. It is there somewhere in the rules reference guide under the damage/horror cards I believe.
Are you talking about this:
When an effect instructs an investigator to flip one or more
Damage or Horror faceup, he selects those cards at random from
among all of his facedown Damage or Horror cards.
So when it says at random, the common practise is to shuffle and take the top one?