FormerlyHemlock
Hero
Let's say you are in the enemy hideout and entering the sleeping chambers. You encounter some enemies there just went to sleep (if it's day, it's the night shift). You kill them and close the door. Now I find it reasonable to say there could be up to 6 hours in which nobody enters the chamber again (after that the guards looking for their shift replacement for example or the next group wanting to sleep comes). So a short rest is quite likely to succeed, a long rest would probably never succeed, however.
In the official modules dungeons often have rooms were enemies are very unlikely to enter in general, too. Even if there are patrols, they would usually patrol the hallways rather than entering every single room (unless they notice something suspicious). Keep in mind that they have probably been patrolling every day for months with nothing happening, so they won't be super alert by default.
And if hearing someone behind a door requires a successful DC 15 perception check then hearing someone behind two doors is probably at least very hard (DC 25), so very unlikely to happen.
And talking about non-inteligent creatures. If you block their path (e.g. by closing a door), they are unlikely to enter. And many of the creatures (e.g. spiders) are more lurkers, waiting for victims to come rather than they are hunters, actively hunting for prey.
I find all of this quite reasonable and it makes the dungeon crawling a lot more enjoyable.
If you kill a sleeping creature, that falls under the "surprise round/ambush" caveat that I mentioned earlier. I find it believable that you could have 6-8 or more fights in a day if they are mostly surprise ambushes, because the enemy won't have time to shout, "To arms! to arms!"
However, I think you are vastly overestimating the difficulty of making noise, especially when there are "patrolling" guards. DC 15? More like DC 5.