DND_Reborn
The High Aldwin
I would think it depends on how the passive check is being used. Passive checks are used in two instances:
1. instead of rolling the same ability check repeatedly (the finding secret doors over and over again part...)
2. when the DM wants to secretly determines without rolling dice (noticing a hidden monster part...)
In the first instance, guidance can't be used for the passive because it applies to only one check. In essence, the PC would perhaps use guidance on one specific check first (a normal active ability check) when the activity starts, but then cannot apply it to any passive checks. Since guidance uses your action, you couldn't even cast it each round and then make the check since both require your action; so it wouldn't work with passive checks IMO.
Now, I could see guidance perhaps being applied to instance 2 above, where the DM doesn't want to disclose information but the situation might be more "active". For example, a PC might be scouting and think a monster is hiding in ambush, so asks to make a Wisdom (Perception) check. The DM asks for the character's passive score, and the player says "I want to roll my guidance d4, ok?" The DM says "Ok, go ahead and I'll add it to your passive score."
However, in the second instance, often those uses are due to the PC not "actively" using the ability, which is why it is passive. In such cases, guidance wouldn't work IMO.
That's how I run it anyway.
1. instead of rolling the same ability check repeatedly (the finding secret doors over and over again part...)
2. when the DM wants to secretly determines without rolling dice (noticing a hidden monster part...)
In the first instance, guidance can't be used for the passive because it applies to only one check. In essence, the PC would perhaps use guidance on one specific check first (a normal active ability check) when the activity starts, but then cannot apply it to any passive checks. Since guidance uses your action, you couldn't even cast it each round and then make the check since both require your action; so it wouldn't work with passive checks IMO.
Now, I could see guidance perhaps being applied to instance 2 above, where the DM doesn't want to disclose information but the situation might be more "active". For example, a PC might be scouting and think a monster is hiding in ambush, so asks to make a Wisdom (Perception) check. The DM asks for the character's passive score, and the player says "I want to roll my guidance d4, ok?" The DM says "Ok, go ahead and I'll add it to your passive score."
However, in the second instance, often those uses are due to the PC not "actively" using the ability, which is why it is passive. In such cases, guidance wouldn't work IMO.
That's how I run it anyway.