HaBO: She Witnesses a Crime at a Ball

This HaBO was sent in by Jamie, who wants to find this historical romance:

I am hoping to get some help on finding a book that I read over 10 years ago. It was a hardback book that I got at a used book sale. The cover was a reddish color, with a photo of a woman in a red or gold dress facing away from the camera on the back. It’s a regency-era historical romance, but with some adventure elements.

The details are pretty fuzzy, so they may not be remembered exactly correctly.

The main character has red hair and while attending a ball, she witnesses some sort of crime or break-in that the hero is somehow involved in, leading to a sort of accidental kidnapping situation. They end up traveling through some sort of smuggler caves or cellars, and are being pursued by some other bad people. I believe near the end they end up eloping in Scotland or Gretna Green.

My guess is that it may be part of a series, since it mentions in the beginning that the heroine has at least one happily married sister, but is not particularly interested in finding a match.

Sound familiar?