Last night on the way home from work, I stated losing pressure from my driver side rear tire.  It was dropping about 2 psi per mile.  When it reached 6 psi I pulled over and used the air compressor and flat sealant provided with my car to re-inflate the tire which carried me another couple of miles. I repeated this three times before reaching home.  This morning I pulled the wheel and tire and took it into the dealership for a replacement.  The flat was caused by a snapped off section of a utility knife blade.  I have little doubt I picked this up at work when I parked near the door where deliveries are made. Lesson learned.