If, hypothetically, I had both a rogue PC and Adaon as my companion, would my perception be higher than if I just had my rogue PC? Or does it just calculate based on whoever in the party has the highest?
If I understand the equations correctly, each character makes an INDEPENDENT detection attempt based on his or her own detection ability. Thus, the detection doesn't stack but having a Companion doubles the chances you make to find a secret.
On a far less notable point, the detection check is based on where the character is standing, thus, a main character and a Companion might have different search radius if they are far apart for whatever reason.
We're splitting hairs over word choice but that's the nerd in all of us
Having a Companion DOUBLES the OPPORTUNITIES to detect a secret as both the Main Character and the Companion make a check every few seconds as noted above.
The strength of each check is determined independently. So in a mathematical sense, the statistical chance of success is only doubled if the Companion has the same detection level as the Main Character. If the Companion has a better detection rating, the true chance is more than doubled. The opposite is true as well. If the Companion has a lower ability, than it will be less than doubled. That presumes both the Main Character and the Companion have a non-zero positive chance to detect the secret in question.
We scribble out an equation that would map out the probability of finding any given secret based on the above, keeping in mind that it would be a binomial probability operating independently for both the Main Character and Companion... but yeah, people tend to fall asleep around two paragraphs ago.
Oh... keep in mind the bonus/malus tossed into the mix. That can undermine the use of a Companion to find secrets.
Each secret door has a level (some listed on Wiki maps, some not). There's a 3% per level added, or subtracted, if the Character is above or below the level of the door. The bonus is maxed at 15%, but no cap in the other direction. See Wiki section on that.
Given Companions level up at a lower rate than the Main Character, that level difference might be a hidden penalty that erodes any higher Finding Secrets ability of an Companion.
For combat Warriors, which tend to have the lowest Find Secrets rating, Adaon might still be useful in finding secrets, but I would still encourage the use of Scrolls of Detection.
For Characters with a moderate to high Find Secrets rating, the Companion's contribute might come out in the wash.
For the stats nerds out there:
A 11% chance to find a Secret has a 90% of finding it after a minute (20% gives you 99% at 1 minute). So as long as there's a small chance of success, you'll likely trigger the Secret fairly quickly, often just by playing though the dungeon once or twice. Binomial Probability raises it's ugly head again.
It's the same math as Rare Drops, just with different values. viewtopic.php?f=4&t=10279&hilit=rare+drops+chance+probability