I had to run an errand. So I left (it only took about 15 minutes). I came back and found poor Buddy sitting in the corner at the top of the stairs of the hallway of my apartment. He looked terrified. I scooped him up, covered him in kisses and apologized. I hadn't realized he snuck out. Thank God I have an enclosed apartment building or he would've been long gone.

I feel awful. I gave him a handful of treats and then fed him.

Lesson: In multi-cat households, ALWAYS take inventory upon leaving and arriving.