It wasn't the stairs.
Even the knee that doesn't hurt makes popping sounds loud enough for people to hear...strangely, though, it isn't the crackle pop that hurts. Weight on my right knee at any angle other than 0 degrees generally causes some level of pain now.
Idiot me, I did the circuit training with command PT. Running, jumping jacks, squats, push-ups, side leg lifts....all of these things made it bad enough yesterday that I had to go buy some tylenol (since the moving emptied the small bottle in by backpack) so that I could ignore the constant low ache coming from my knee.
Today, at least, I have the duty so no PT for me.