I would be concerned with a loner mouse having arrived at Patrol Guard rank without an interest in internal politics. Yes Ptl Guard should be independent and capable, but they've also got to have the highest level of trust from the Matriarch.
I don't see that the Instinct is much of a problem. You will have to imporvise what is in the scene rather than expect the GM to describe each detail. In fact, that is a selling point over D&D. Rather than play a game of 'mother may i' by asking, "Is there a rock over here," "Does that stream bend over this way," "How tall is the tree?" you can declare points of setting and props. It makes the entire game better when you do so. Why wouldn't you declare the aspects of the scene that can be used by your Instinct?
It might be true that in some scenes the Instinct doesn't fit, but that isn't a problem. To earn the reward, you need only use it once in a session. If you are truly trying to use it in every moment of description, it might become a bit cliche.
Another factor in Instinct is creating a statement with a trigger that the GM can play with. In your current belief, there is not much a GM can do to trigger a response. Image if you swapped the current Belief to become an Instinct: Never fight an animal which can be trapped. In such a case, every time that the patrol is faced with a creature (mouse or animal), you could start to suggest ways that the patrol could work to trap it rather than fight it. That uses your Instinct and allow the GM to trigger it by introducing an animal.
If perhaps you shifted the current Instinct to a Belief: The Guard must understand and use the environment advantageously, I suspect you could have lots of fun with that. I don't immediately see ways that you could play against it, but I'm certain there are some scenes that pausing to understand and use the environment would be detrimental. This would allow you to use a large variety of skills in support of your belief too. Right now, just Fighter and Hunter get spotlighted, but with the Belief I wrote you could spotlight Weather Watcher, Hunter, Instructor, Scout, Pathfinder, Cartographer, Orator, Persuader, Deceiver, Nature (Mouse), Stonemason, Carpenter, Smith, Armorer, and probably more. See, in each of those skills you can easily describe how Grint understands and uses the environment while using the skill--this includes when using those skills to help others.
Gaining reward for Playing Toward or Playing Against the Belief is more difficult in-game. You should make the Belief broad enough that it can be applied more easily. When an Instinct can be triggered by the GM, you help to make a more collaborative game; other players could also act to trigger the Instinct.
-- The Guard Prevail --