I found where the critters were hiding. And from the looks of things, it has been for a really long time.
I had some boxes piled in the corner. NYC apartment, moved in 2 years ago, usual reasons... The bottom box has holes for handles. It appears the holes were used as a front door for the mice, and some breeding occured. With the addition of my bird cage nearby, they had an unlimited food source. I even cleaned it for them and kept it stocked.
The box is now sitting on a table outside and I am debating whether I should just throw everything away, or go through it and salvage what I can. As cute as mice are, they really freak me out. Yes, I am a wimp when it comes to rodents.
As an aside, I did some errands on my motorcycle today. First time since November. Sometimes I forget how much I love riding.