Checking the fuses in the passenger compartment behind the glove box, I decided to pull out the AC/Heater filter that is visible there.
This is what I found:

Click image to enlarge

Click image to enlarge

Doubt that is a mouse

just stuff coming in through the vents a sign that it has not been changed/checked for some time, would check your air filter if not done so already
Peeps don't bother with the pollen filter, just serviced my Subaru (a recent purchase) and all other filters have been changed but not the pollen filter it nearly looked as bad as yours.
Thing is on the RRS it is a 2 minute job to check and change if needed.

Well, the filter had been chewed away to a large extent. Don't think leaves would do that.

And besides what you see is only the smaller leaves, big ones 2" x 6" were atop that. How do they get in? .
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Come for a visit, Victoria is beautiful!

Looks nibbled to me

The patch on the left is oval and nibbled out by they looks of it

Get a trap in there set with peanut butter and see what happens

if you just cover the filter, what else may get eaten, as if it is a moose it's still getting in

the big hoof prints on the bonnet may be a give away as well

I hate to tell you, but a mouse may still get through that grill. They can squeeze through holes as small as approx 8mm.