The pads are dry.

This is still one of the oddest stories I've ever heard. Don't you *have* to touch your bag just to get it to roll into the xray machine? I've even been yelled at because the TSO thought I was going to walk through the WTMD before pushing my own bag through.

Did you hear any beeps as you walked through the metal detector? Could you have been randomly selected for additional screening and that might've been the reason you got the swab?

This is a prime example of when it'd be great to hear a TSO tell us what might've happened.