Here is how it could work:
- There will be occasional patches of hide grass on the map. They will be about as rare as current speed-boost objects, maybe even rarer, and they will be somewhat larger in size than they were previously (maybe like 6-8 tiles in diameter). Certain appropriate animals can be spawned inside patches of hide grass when it is loaded or just wander into it, it can also be empty most of the time. Animals which are inside the grass are invisible when they are not in combat.
- When a character sees the grass he rolls perc*explore to have a chance of detecting if it is empty or not. If he detects some animal inside, then he sees it as usual. If he failed detection or there aren't any then grass appears empty. If a character approaches/enters the grass then he automatically successfully detects what is inside.
- Thus, early these spots will represent potential danger of approaching them, as there could always be angry bear hiding inside. But they also will not be too unfair as there are two ways of avoiding this danger: by raising your perception*exploration score or simply by spending time circling them around. And at a later point of character progression they can become sought after hunter spots, where there is higher chance to encounter the game, compared to random wandering
Also, it might or might not be possible to use grass patches for hiding characters as it was initially meant and it is not the point of this idea.