How do Sophie and Langdon determine what to do after they have the key?

Sophie and Langdon decide on their next move when they notice that the key looks recently cleaned and smells of rubbing alcohol. Langdon requests to see the pen light. There is an address written in invisible ink on the key. Sophie instructs the taxi driver to take them to the address. Although that still doesn't solve the mystery of what the key opens, it does give Langdon and Sophie their next big break in the case.