Some categories of checks follow special rules. The most notable are flat checks and secret checks.
When the chance something will happen or fail to happen is based purely on chance, you’ll attempt a flat check. A flat check never includes any modifiers, bonuses, or penalties—you just roll a d20 and compare the result on the die to the DC. Only abilities that specifically apply to flat checks can change the checks’ DCs; most such effects affect only certain types of flat checks.
If more than one flat check would ever cause or prevent the same thing, just roll once and use the highest DC. In the rare circumstance that a flat check has a DC of 1 or lower, skip rolling; you automatically succeed. Conversely, if one ever has a DC of 21 or higher, you automatically fail.
Sometimes you as the player shouldn’t know the exact result and effect of a check. In these situations, the rules (or the GM) will call for a secret check. The secret trait appears on anything that uses secret checks. This type of check uses the same formulas you normally would use for that check, but is rolled by the GM, who doesn’t reveal the result. Instead, the GM simply describes the information or effects determined by the check’s result. If you don’t know a secret check is happening (for instance, if the GM rolls a secret Fortitude save against a poison that you failed to notice), you can’t use any fortune or misfortune abilities (see the sidebar on page 449) on that check, but if a fortune or misfortune effect would apply automatically, the GM applies it to the secret check. If you know that the GM is attempting a secret check—as often happens with Recall Knowledge or Seek—you can usually activate fortune or misfortune abilities for that check. Just tell the GM, and they’ll apply the ability to the check.
The GM can choose to make any check secret, even if it’s not usually rolled secretly. Conversely, the GM can let you roll any check yourself, even if that check would usually be secret. Some groups find it simpler to have players roll all secret checks and just try to avoid acting on any out-of-character knowledge, while others enjoy the mystery.