I suppose that opening a lock creates some noise. I'm wondering how to houserule a silent disable device check. My idea is imposing a -5 penalty to the check adding that if you fail by 5 or more you make noise instead. Due to the fail consequence you cannot take 20 and, assuming that, if you are trying to make no noise, you are also someway in danger/stress (to be discovered) you can not take 10.
Some suggestions comments?
In my mind, picking a lock stealthily would still allow you to take 10. (if you could normally take 10 on the lock in general.) Since you could theoretically be in danger/stress when you are picking ANY lock since you could be discovered.
Same thing for taking 20. Picking the lock itself doesn't have a consequence for failure. So you could take 20. But picking the lock would possibly let the people on the other side know you are there, and then they can open the door on their own.
However, I would assume that picking a lock would be audible with a DC 10 (or 15) perception check, with the modifiers for distance of any possible listeners taken into account.
Adding noise as a penalty for failure or changing the DC to reflect trying to do it silently is houserules territory and that part of the discussion belongs there.
Attempting to do activity silently sounds to me like a Stealth check. Depending on the nature of the mechanism, it could opposed vs perception, maybe a slill ossiciated with the maker of the device, or vs a fixed DC. For example, if a counterweight is being disengaged and will make a noise when this occurs, stealth isn't going to prevent that, although the noise itself may not be heard by an observer. This could be very situational and or plot driven and really gets to a matter of play style.
In general, breaking down an activity into multiple checks is the way to go, but be very careful of how the probability of multiple events decreases success dramatically.
In the past, I've always ruled doing anything quietly/sneakily as make a Stealth check to do so, regardless of surrounding circumstances. Depending on circumstances, trying to pick a lock quietly might impose a -5 penalty, taking 10/20 might not be permitted, and/or even if they successfully open the lock, a person on the other side might have better Perception than the character Stealth, so they could be noticed even if they succeed.
I like your idea of putting a penalty on the Disable Device check as a result of trying to do it quietly. Good thinking. If time isn't a factor, though, I wouldn't disallow taking 10 if it was otherwise permitted.
I think than resolving it with a stealth check is not the right way. Stealth is about "hiding and moving silently". I interpret moving=move action (so walk or also climb).
Making other specific actions without noise (like opening locks) should be covered with a penalty on the action check, not adding another check. I think if I'm quite good in disabling device I would have more chance of making it silently than a character having a lower disable device but an higher stealth check.
That's fair. There are reasons to do it both ways. I personally prefer using the Stealth check since it also allows me to extend it to other things, like Stealth-casting. It does also cover "avoiding detection," as a generalization, meaning it can be considered applicable. That, and picking a lock ain't always a quiet operation. Lots of little clicks and scrapes to them as hears.
There is another way to do it. For doing Disable Device sneakily, treat is as an opposed check. Your Disable Device check against their Perception check, meaning they have a chance to notice it regardless of the success/failure chance.