My husband said he has seen this weekly snd has been cleaning it up. So it's happening more frequently than once a month. It looks like coffee grinds

And they aren't little bugs? Are they anywhere else in the house?

Do you have someone in your home that plays sports on turf? My kids bring home tiny pieces of what I assume is rubber after playing on turf.

Are there cockroaches in your area? We don't really have many in my area so I've never seen one or lived with them, but when I was looking for pics of mouse droppings just now, there are pics of cockroach droppings (I guess I never realized they left droppings) that look exactly like that. Not sure if they would hang out by a furnace vent or not. Oh, but I see maybe dog food is nearby?

Yikes, I hope it's not cockroach droppings! After looking things up, I also thought it could be spider droppings. I didn't realize they leave droppings either.
I don't see these anywhere else in the house. It's always scattered in the same place.

Is the dog food in that area? I see the water, wondering if you feed the dog right there? If so, maybe feed in another place and see if the dropping stop or move.

I clean my house all the time. Now I'm worried and wondering how to find this critter? I'll clean my cabinets tomorrow. I'm looking up what resembles roach droppings praying it could be anything else!

Okay, they came back today... and a few were moving! They're bugs. I'm not sure what kind though or why they're always in that one spot.

glad you figured it out. I'd call a pest company for advice.