Genesis 42:9

Ye are spies] Egypt was always liable to attack from Asia, and fortresses were built along that frontier to repel invasion. By suggesting that they were foreigners who were spying out the nakedness of the land, i.e. how far it was open to attack from hostile nations, Joseph had an opportunity of enquiring about his family. We may believe also that, though well-intentioned towards his brethren, he sought to bring their sin home to them.