A MOUSE IS IN MY HOUSE

I have a mouse (and recently, a rat problem) because I own ducks. I made the mistake of leaving their feed around in the garage everywhere. The mice have since settled in but I have took away the food so I have noticed that they are dwindling. I haven't spotted a mouse or rat in the garage for a very long time. However, at one point a rat took up residence in my garage ceiling, or first floor floorboards.

I also recently noticed that they have been absolutely ****ing tearing through the insulation around my water heater. This is real bad!! They are clearly in the walls now.

What's really, really, bad, is that the last time I had a mouse loose in my kitchen/house was because I caught it in a glue trap, thought it was so cute I couldn't kill it. So I put it in a cut-out gallon of water jug and left it there. Well. He got out, easily. Tonight, I just noticed a small brown shadow just scurry across right outside the kitchen and just disappear into this grate that expels the cold air from the house when the heater is on.

Now I am really ****ed. They are in the ventilation? I don't even know how. I have no idea where these vents are going, but the fact that it's somehow in the ventilation is really ****ing bad.

In the past, the only nests I've noticed that they've had were some kind of nest in the garage, but I just don't know where, and then for a very long time they had a nest under the house in the garden. It was chewed into the foundation of the outside of the house and I just stopped bothering with it after a while. But HOW did they get into the ventillation. That is bad. The gas heater has these insulated pipes that go into the house and they just absolutely have been chewing through some insulation, given that there are chunks of it on the floor I haven't dealt with yet above the heater. But HOW are they in the ventilation itself?

WHAT DO I DO THIS IS BAD. I could easily just put something heavy on the grate that expels air, but like WTF. Does this mean the mice are desperate for food that they've come searching up here? For the last 4ish months of having a mouse/rat problem - I've never SEEN them come up through that and I've never seen signs of mice in my kitchen aside from that time I KNEW one escaped.