He could have taken a spare key.

他原本可以带上一把备用钥匙的。

《柯林斯英汉双解大词典》

We can't lend you a spare key.

我们不能借给你备用钥匙。

youdao

Can I have a spare key made?

我可以再配一枚备用钥匙吗？

youdao

Using the spare key that he carried, he unlocked the front door and entered the house.

NEWYORKER: The Autobiography of J.G.B.

While Mr Coats was in the pub, the BMW car which he allegedly took from Mr Burns, was driven off by a friend of the bar owner using a spare key.

BBC: Lynda Spence trial: Murder accused 'threatened bar owner'