Exploring an abandoned house

You find an old, abandoned house when hiking through a forest. After a cursory look around, you see that you're all alone. Should you explore? Yes! The door creaks open. Screeeeeeeeeeech.

Dust motes dance in the little bit of sunlight streaking through the dirty and broken windows. After setting your backpack down, you rummage around and find your flashlight. Taking it in hand, you explore.

There's not much to see on the ground floor except a bunch of dust, broken glass, and evidence of animals living there at some point. Finding the staircase, your left foot tests out the strength of the first stair, then the second, then the third.

When you get to the top of the stairs, you notice that it's brighter up there. Footprints are in the dust on the floor. You aren't sure if the person belonging to the footprints is still around so you call out. "Hello?" No answer.

Suddenly, a bird flies out of nowhere, startling you. When you fall into the wall behind you, an envelope falls loose from somewhere. You bend down to pick it up and find a stack of cash inside. From the looks of it, it's really old.

What happens next? How did the money get there? What are you going to do with it? Where did the envelope come from? Who left the footprints?

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