I disagree, because according to that, you can organizate a rebellion OOG, and joining the underground at the same time. 20 nobles joining the underground, being at the capital and starting a rebellion at the same time is not fair play, in my opinion. The ruler would go mad, because he checked the underground a few hours before, because there were a lot of untrusted nobles sitting and rallyiing in the capital, but "nobody" was there.
Does any serious, well-organized rebellion consist of people sitting around in the underground for weeks, waiting for just the right moment to rebel? I could be wrong on this one, never having organized a rebellion myself, but I understand from people that have done it, that the most successful rebellion pretty much consist of what you describe: Lots of people joining the underground as close to the last minute as possible so that they aren't discovered. It also prevents the ruler from catching on to the rebellion, since quite a few rulers have spies in the underground anyway. They would know when the rebellion limit was getting close.
Also, secret police can only be used once per turn. So the ruler could, at most, have checked only one person.
