In "In the Midst of Alarms" (1894) by Robert Barr, Yates is flirting with a rural Canadian girl in her kitchen.
“No such motive drew me into the kitchen. But I will tell you. You shall have it from my own lips. That was the reason!”
He suited the action to the word, and kissed her before she knew what was about to happen. At least, Yates, with all his experience, thought he had taken her unawares. Men often make mistakes in little matters of this kind. Kitty pushed him with apparent indignation from her, but she did not strike him across the face, as she had done before, when he merely attempted what he had now accomplished. Perhaps this was because she had been taken so completely by surprise.
I can't get what's the significance of "at least" here. And was his mistake that he didn't expect her anger?