HRM's & am I REALLY burning that many cals?

OK - I wear my HRM when I'm working out & that includes at work when I take lunch or a break & "do the stairs"...that's up & down 30 floors at least twice during one break...I do that twice a day - sometimes 3 times if I have the time. This morning I forgot to to STOP the HRM from it's counting process until 15 minutes after I had returned to my office. I know we do burn calories while we are resting & sedentary, but I looked at the HRM at the end of of my second time on the stairs & it shows 264 cals...when I remembered to push the STOP button it was 15 minutes later & the cals said 472. Did I really burn 208 actual cals after I got back to my office? I know my heart rate was still elevated for several minutes, but is that really a true count or not??