The current system is a simplification. While I don't think it would be that hard to have the game keep track of whether each individual animal is aware of your presence, providing a UI feedback of it would be tricky. One effect of the simplification is that you've got an indicator telling you whether you're undetected or not, which is useful, but not realistic. With a relation with each individual animal you can't have a unified indication, but would have to examine each animal of interest to judge whether it seems to notice you or not. Unlike real life, where that feedback is automatically fed to your brain from your sensory organs, you'd have to somehow query the game for it. One tedious way would be to look at the creatures of interest every turn, another might be to mark an animal as being of interest and have its assumed status displayed where the current sneaking status is, while a third might be to have different animal symbols or symbol modifiers to indicate their perceived state.
But yes, a more advanced system that's not annoyingly key/click intensive would be more immersive.