A while back I wrote a post about why I disliked the Ability Check mechanic recommended in Old School Essentials and analyzed various alternative methods. I wasted a lot of words on it. Here is a graph that shows you the differences between several methods and you can draw your own conclusions. As you view it, keep in mind that in most cases a first level character with a particular class skill (e.g. hear noise) will succeed 33% of the time (a 2-in-6 chance as indicated by the gray horizontal bar).
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After playing around with skill rolls for a while at the table, my conclusion is that, because every method other than d100 under is consistently better (unless the stat is abysmal) than a 2-in-6 chance, a roll under ability check with a d20 should only be used for skills that "anyone could do" and which are not a defined class skill. And even then I would probably ask for a 1-in-6 chance roll or not even bother with the dice.
In any case, class skills that seem like anyone could try them, i.e. "hide in shadows," need to be read as potent rather than mundane skills. In other words, the hide in shadows ability allows a thief to disappear in a brightly lit warehouse (in the shadow of a crate perhaps) or even behind another, larger character in a hallway. Characters without this ability couldn't even attempt such subterfuge, and should only be able to hide in shadows when it's really obvious that anyone could do it, e.g. standing in a side alley at night.