The first thing you could do is make objects create clattering and noise when they land after you throw them, based on the floor terrain and the weight/material of the object.
Zombies and the like already investigate loud noises, so a clattering tin can would work wonders.
The second thing you can do is either have a stealth skill, or have various stealth styles.
If the former, then the higher you get, the more techniques you unlock. Could tie lockpicking into it, and silent crowbarring. As your skills go up, your footsteps get quieter and you gain debuffs to enemy sight ranges if you’re next to walls or objects.
If the latter, then I guess you could have stealth techniques that focus on different things. Ninjitsu stealth would be different than say, navy seal stealth.