‘ And it was told the king of Jericho, saying, “Behold, there came men in here tonight of the children of Israel, to search out the land.”
Someone, possibly one of the ‘guests' made suspicious by their questions, or possibly a watchman at the gates who noticed where they went (Rahab's house was on the city wall), sent a report to the local king about their visit. They would be given away by their clothing, their looks, their dialect and the workings of suspicious minds. Indeed spies had probably been expected and they would be on the watch for them, for news would have come through about the total defeat of the Amorites and that a large army was waiting to cross the Jordan once the floods had subsided.