I immediately froze, and with a shaky voice, said... "where"?? The answer... "Under your foot". So, I look down, and sure enough, there's a dead little mouse right there. I can't remember exactly what I did at that point, but I think it involved a little bit of screaming, panicking, almost crying... and I do know whatever I did was embarrassing.
I went straight home, scrubbed my feet and flip flops, and vowed to watch where I step every session from now on.