take (one) by surprise
To encounter or otherwise engage something or someone who is not prepared or on guard; to shock or startle someone or something by one's sudden appearance or action. You took me by surprise when you showed up at my office with flowers this afternoon! Our soldiers descended upon the enemy encampment just before daybreak and took their troops entirely by surprise.
take somebody by surˈprise
happen to somebody unexpectedly; surprise somebody: His decision to retire took us all by surprise.