Well to be completely Honest you can hear damn rats and mantis, when you stay in enco for too long. Why would you not hear a player?
That.. Is.. A good point..
Hmm, well I guess they are usually groups of mantis and rats, making noises to eachother, so part of my argument still kinda applies.
Being detected as a sneaker before you have even spawned is a horrible idea though, it basically renders the sneaking useless, as people or groups will know to search for you, at the very least sneakers should bypass giving warnings.