Using Luck on Secret Rolls

Luck is probably my favorite GURPS advantage.  I've said many times that if you don't take Luck, and your character dies, it's your fault for not taking Luck.

For those who don't play GURPS or Dungeon Fantasy RPG, the way Luck rolls is that, once an hour you get to reroll a bad roll, twice, and then take the best of the three attempts.  (And there are more expensive versions that work more often.)

That's simple enough, if the player makes the roll.  You try to parry the boss monster's attack, you roll an 18, you know this will result in both a failed parry and a fumble, you don't like that outcome, you announce that you're using Luck, and you hope to roll better on one of the other two tries.

It's a bit more complicated if the GM makes the roll in secret, but the player knows what the GM is rolling for.  Like, if you have Weather Sense and want to predict the weather for tomorrow, and it's important to get this right, the rule is pretty clear.  "When the GM rolls in secret (e.g., to see if you notice something), you may state that you’re using your Luck ahead of time, in which case the GM will roll three times and give you the best result."  So, if you happened to roll great the first time, you didn't need the Luck and wasted a use, but better safe than sorry.

Where it gets tricky is when the player doesn't know the GM is rolling dice at all.  For example, the Per-based Traps roll to detect a hidden trap is a passive roll: it happens whenever the PCs come near a trap.  But if the GM tells them that he's rolling to detect traps, then if the PCs fail that roll, they will probably immediately switch to an active roll to detect traps again, or go the other way to avoid the trap they shouldn't know about.  So, they need to not be told.  But if you don't tell them, how do they benefit from Luck?

I see a few options here:

- If you don't know the GM is rolling, you don't get to use your Luck on that roll.  Harsh, but simple and following the rules.

- Tell the GM to always use Luck on secret rolls.  Simple and following the rules, but it means you'll probably burn your Luck on rolls that don't matter very much, or rolls that would have succeeded without it.

- Leave it up to the GM's best judgment.  "Hey, if a secret roll goes really badly, use my Luck."  Though this implies that Luck works if you don't know about the roll, which may or may not be true.  So this requires a generous GM.

- Give the GM some specific rules.  "If I fail on a secret roll that would have succeed on a 14, use my Luck."  Or "if I critically fail on a secret roll, use my Luck."  This implies that Luck not only works if you don't know about the roll, but that you can trigger on the details of a roll you don't know about.  This requires a more generous GM.

The Whiterock game features a PC with both Luck and really good Perception who likes to scout ahead, so this will come up constantly.  I need to make a decision before our next game on Friday.