Alertness is a special card, marked in gamemaster colors, so you will be reminded of its presence. The card assures that a hero searching or examining an area will find or notice something (information, an item, an ambush) as long as it is there to be found, whether he makes his find roll or not. After a player puts the card in his pool, you activate the card whenever the character misses something of importance.
Example: If the Yellow Crab were having an especially bad day when he arrived at the scene of a murder, he might have missed his find roll to discover the Sumerian dagger stashed in the garbage can beside the desk.
Chris, frantic to find a lead of some kind, drops the alertness card on the table. Because there is something to find in the room, the gamemaster takes the card, puts it in the discard pile and mentions that the Crab spots a glint of gold in the garbage can — a rather peculiar sight.
If there had been nothing to find, the card would have remained out until the Crab missed a find roll, at which point the card would be taken and a clue given.