When players roll their own perception checks, they learn something from the number on the die roll. Players with bad rolls know that their search may have missed something; players with great rolls may trust the game master’s report that they found nothing.
As a game master, you can make perception checks in secret, but players hate this. Some of the fun of the game comes from rolling dice. If the GM rolls for your character, you start to feel a loss of ownership. You feel like a bysander watching the game rather than participating.
Who should roll perceptions checks?
Many players take pride in running their character without relying on any metagame information. These players can roll, obviously blow their check, and press ahead knowing that if a trap awaits, they missed it. If you have such players at your table, let them roll their own checks. Still, even for these players, knowing the rolls can rob the adventure of some sense of peril and mystery.
Some players take the unearned information that comes from the number on the die and they use it to make choices. For them, remind them that their characters don’t know they blew a search check, so the characters lack any reason to repeat the search. If they keep searching anyway, roll the second check for them, out of their view. And if you simply ignore the second roll, no one will know.
If you want a game that emphasizes a sense of challenge and risk—or you have a table of unrepentant metagamers, I suggest an occasional roll substitution.
Tell the players, “Whenever you make a perception check, I will secretly roll a d6 and a d20. If I roll a 1 on the d6, the information I tell you will be based on my d20 roll rather than on yours.”
With this approach, most of the players’ rolls still apply, so players remain connected with the game. The visible roll gives the players a fair sense of how their characters performed, just as you might have a sense of your performance on a real-world test. But players can never feel certain that a 19 on the die means they found everything to find. And when someone rolls a 1, but sees a glimmer on the horizon, it may be more than a mirage.
If you only ask for perception checks when something can be noticed, the checks will put players on alert. Once or twice a session, when nothing can be found, you should call on players to make a Spot or Perception check. No matter what the roll, frown, shake your head, and tell them nothing. Not only will this unnerve the players, but it trains them to avoid assuming that they must have missed something.