1, The question seemed to catch him off his guard.

2, The lawyer's seemingly innocent question caught the witness off his guard.

3, This catches William off his guard.

4, He has been off his guard recently.

5, Surely what a man does when he is taken off his guard is the best evidence for what sort of man he is. C.S. Lewis 

6, The best way to ask the boss for a rise is to catch him off his guard and just slip it into the conversation.