Is there any particular reason why the coins in the sewers of Super Mario Sunshine are questions marks instead of coin icons? Is it a glitch, or is their significance to it? Maybe (a lame) gameplay reason to keep you guessing what's underground?
This isn't specific to coins - any items in the sewers that you can't see are question marks, and you'll have to judge by the sound (among other things) to determine what you pick up.
Coins are by far the most common, but you can also run into 1-UP mushrooms.