Simple rules to know if PCs spot the things.
There is always a DC to see something that is not plainly obvious in the environment. It should probably be no lower than 10, and if it is, maybe it's plainly obvious and not worth a check, but exceptions could occur (running through an area, you could overlook that DC5 thing).
If the PCs are just kinda passively going through an area, they have a check score equal to the party's cognizant members (don't roll anything unless they have elves). Roll a d3 for each elf in the party and add that result on top. If the party meets the DC, they notice the thing without having to try too hard. Easy.
The next part doesn't use the DC thing. You could find a way to pretty easily though.
If the PCs are actively searching a room, roll 1d30 (secretly). If the D30 rolls at or under the amount of PCs helping out, the group spots the thing. Add in 1d5 to the roll-under score if any PC elves etc. are present. The party can spend Luck if they want. Searching an area this way takes half a turn (5 minutes), which might matter if you have random encounters or wandering encounters.
Be prepared to consider some things:
Do henchmen count for the active roll? Maybe if they have the right job or species for it. Dogs could help spot stanky things. Dwarves could help sniff gold. I'd probably let most dwarves just know if something is fishy with architecture. After all, no roll is the best roll.
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