In order to get any frost to develop, you need moisture in the atmosphere. Late Sunday night into Monday morning fog developed as moisture became trapped in the lower levels of the atmosphere. Since temperatures were roughly ten degrees below freezing (32 degrees), freezing fog developed. At least one or more nights of freezing fog is a perfect recipe for the development of hoar frost.
Conditions may be very similar Monday night so the possibility of not only dense fog to develop, but freezing fog, is there as well. Be on the look out on the roads Tuesday morning, but also for those frosty trees!