This is absolutely bizarre. I've been clumping around at work in my bike shoes today and just now walking along the corridor noticed a clicking sound coming from my right foot. "Hello, that sounds strange," I said. Tap-tap-tap...clunk! and the cleat fell off my shoe. I didn't realise that the mud from a bike ride a month ago was actually structural. The bolts themselves have vanished.
I have several spare bolts at home to reattach things, but now have to see if I can manage to ride home with one bolt per shoe! I was planning to move the cleats back a little anyway but hadn't planned it quite like this.