It probably occurs to most people when looking at the OD&D thief abilities for the first time that the chances of succeeding when using any of these said abilities at 1st level is pretty slim! At first glance it seems quite harsh on the player.

Imagine a 1st level apprentice thief who is a dwarf (so gets some bonus) is attempting to follow an unlawful merchant back to their hideout to retrieve stolen goods. The thief wants to remain hidden and silently so the relevant skills would be the ability to Hide in Shadows and Move Silently. The apprentice only has a 25% and 15% chance of being successful at these things. The obvious issue is that hiding at night time behind barrels of goods, or wagons and carefully following the merchant from a distance is far easier than sneaking up on a guard without much cover and just a little shadow for concealment. So the system as presented is quite ambiguous.

OD&D Moving Silently/Hiding in Shadows in Solo Gaming

A quick flick through my old rulebooks indicates that this ambiguity wasn't really addressed throughout the iterations of old school D&D. Frank Mentzer does offer the following comments in the Rules Cyclopedia regarding Move Silently: "The DM, at [their] discretion, may modify the thief's roll at any time. When he tries moving silently across a field of dried leaves, his percentage chance would go down, while if he docs so during a loud tournament, his chance will be greatly enhanced." Frank also offers the following advise for Hide in Shadows: "Hide in Shadows (HS): Successful use of this special ability means that the thief moves and remains in shadows, making him very hard to see. While the thief is in shadows, observers only get a chance to see [them] if they look directly at [the thief], at which time, he must roll again; success means that [the thief] remains unobserved. While in shadows, the thief may use his Move Silently ability, but attacking someone reveals the thief." This is all pretty much common sense and offers some clue as to how the referee might handle these special abilities but the ambiguity remains. Most good referees might have no problem making on the spot rulings but what if you were playing solo? In that case a little more robustness is required.

Looking at Frank Mentzer's text closely it becomes obvious (if not already!) that Moving Silently has every bit to do with noise. So, essentially, the noisier the environment the better the chance. In regards to Hide in Shadows, of course, the darker it is the better the chance.

Something like the following could be helpful?

BONUSES/PENALTIES TO MOVE SILENTLY:

LEVEL OF NOISE     BONUS/PENALTY
Silent             -10%
Quiet              --
Moderate           +20%
Noisy              +40%
Very Loud          +60%

BONUSES/PENALTIES TO HIDE IN SHADOWS:

LIGHT LEVEL        BONUS/PENALTY
Bright             -10%
Dim                --
Dark               +20%
Very Dark          +40%
Pitch Black        +60%

Let me know if you think I'm on the right track with this one!