Pee or poop or a combination of both, rodents? That would be my first professional estimation, the rodents are coming there for a reason and now I have to figure out why. What is drawing them to that location? Rats are a little shy while mice are curious, so which is it? Looking at the poop, they look large enough for a rats versus the mouse.
What types or diseases to rodents have and why should you be concerned? Did you know that mice urinate as they run, yuck? So you don’t what any of these beasts in your home.