Where have my keys gone

The other day at 3 o’clock in the morning had a call from a gent who’s comments were "my keys have fallen into the sewage" after interrogating him a bit more it transpires that he had dropped is keys down a street drain. This person lives near central London and it had been raining for near on 24 hours, when this happen the storm drains discharge into the River Thames so is keys should now be nearing the coast of France.