I've seen it for years people complaining about old-school dungeoncrawling becasue it lacks essential rules, one area of complaint is the lack of any universal rules for stealth/sneaking. What a lot of people have missed over the years is there is a mechanic built into the game that covers that ground already: The Surprise Check.
Surprise is covered in the combat rules and a lot of folks can't conceive of using it outside of combat but the purpose of surprising someone is to do something unnoticed before others are able to react. So want to sneak past some guards and resolve the situation with game mechanic, make a surprise check against the monster/NPCs if they are surprised you gain the advantage of surprise and if conditions merit no combat happening, there's no combat you have slipped past the guard.
What about thieves, are they being robbed of their skills? Nope, they retain all the abilities they ever did their abilities at hiding and moving silently are enhancements over what other characters can already do. When deployed along with the surprise/stealth roll the seemingly low chances of low level thieves are much improved without knocking the game out of whack.
What about monsters? Use the rules that are there a few monsters are noted as having increased ability surprise PCs or being difficult to surprise, these creatures are simply as perceptive and stealthy as application of the surprise rules would indicate.
There are a lot of rules in place that just need to be applied more often or to a wider range of situations where they fit well.
Check out this post on using the reaction table in a wider range of applications if so interested.