As Matthew said it would have been lovely to have heard this over the Tube public announcement system "Will the lady who lost the slipper yesterday, please speak with the supervisor. Your Prince Charming awaits." Wonder if it fell down the gap?!
Update - There definitely seems to be a shoe issue at Balham Underground as last night, webponce saw another sign there.
He said "First there was Cinderella, now its Dorothy who has lost her footwear at Balham station. Tomorrow, I'm holding out hope for either Rapunzel's wig, a bail of straw left by some Porcine builders, or perhaps a trail of cookie crumbs all the way back to Morden.
More importantly, who loses a shoe and doesn't immediately report it?"