He's being so genuine and gentle with her that it actually catches her off-guard when he brings up the spare key. It's a clear show of trust, and she's momentarily choked up by it.

At least, until he finishes that thought, and she has to press her lips together tight to keep from laughing over it the same way she had over the misheard petname. He has to know, right? ...clearly not, if he's saying it right out in the open.

"And it's only the first date!" Scandalized tone and all, just to poke fun at him.