“戒备的”的英语？

他从背后悄悄逼近了那毫无戒备的受害者。He had crept up on his unsuspecting victim from behind.他脸部带有戒备的神情.He wears the guard on his expression.由于安全戒备的原因，该次列车在“牛津街口广场”站将不停留。Due to a security alert, this train will not be stopping at Oxford Ci...