He was trying to figure out where he was using the map on a bus stop and approached a nearby police car to ask for assistance.
They demanded his ID which he was not carrying at the time. He explained that it was in his bag at the place where he was staying and they said he needed to be able to identify himself, so they arrested him on the spot and he was not free again until the following morning. As a result he missed a ferry and had to spend the money booking another one.