i wouldn't put a hot pack directly on your skin as you mentioned putting them under your panties because when they come out of the microwave they can be REALLY hot. i wrap mine up in a small towel or piece of cloth. once some of the heat starts to decrease in temp., then you can have direct skin contact.
personally, i do not use the thermacare patches as they can be very pricey and sometimes don't stick as they should. i think if you needed to take a hot pack to work you could probably lean it on your abdomen under your desk and no one would know...this is if you have a desk job. luckily for me i go to class for a few hours a day so i can do without. but i do like them for the commute!