IIRC they home in on the C02 from your breath.

Last year one of the little swines got me in the middle on the instep of my right foot. The next day I was setting off back to blighty from Paters on Corfu, only this time I wasn't flying, I was driving back. No automatic and no cruise control. That's right kiddies, my right foot on the pedal for the next 3-4 days. It drove me potty with the itching.