As I have mentioned repeatedly now, physical appearance is only part of any disguise. The rest of it is posture, voice, accent, all the things that make up an individual’s personality and presentation.
I’m going to make any Disguise roll against an unconscious target a take 20.
I can’t think of (nor have I heard) any way of handling corpses that works better than the current one and does not also break other systems, so I’m going to leave it as is for now.
I'm on the fence with this issue but there are a few points I want to bring up. As stated above, posture, voice, accents, mannerisms are all things that make how good a disguise is. If the voice slips, the disguise slips. If the person walks differently, the disguise slips. A corpse won't be doing any of these things that can break the disguise, so how can they be used against detecting a disguise?
How well a corpse is able to maintain a disguise comes down purely to how good the actual make up and prosthetics would be, then. Even with professional special effects artists, you can still tell that something is "off" about a disguise when you see a mask without post-production modification. You can do some pretty extreme modifications with just makeup alone by knowing how light will interact with a disguise and what shades to use to imply depth. There's no shortage of "extreme transformation" videos online of men who make themselves look like women with just some cosmetics.
With that all taken into account, you'd likely have a much easier time detecting a corpse is disguised cause it's all down to how good the makeup and mask might be. If someone has 30 ranks in disguise, it isn't ALL about how good they are with makeup, but 10 points could be makeup, another 10 could account for physical mannerisms, another 10 coming from voice manipulation.
Now, scripting it so that there is a lesser DC to detect a disguised corpse would likely be a massive pain in the neck, so the easiest solution is probably just to say that the person detects a disguise on a corpse.