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