I can't seem to find any threads about people having a similar issue. Here's what's happening -

Canon 5D Mark III, using SanDisk CF cards, photographing in raw.  I reformat prior to every use.  Photographed for roughly 3 hours, could preview the photos with the card in the camera.  Camera behaves completely normally.  If I let the card sit in the camera for more than 15 minutes (ie traveling from location back to office) something happens to the card and upon trying to import files card shows no images.  Put the card back in the camera, camera says no images.  I can use SanDisk's recovery software and get the images off the card and they are fine.  This has happened twice in one month to two completely different cards. 

Anyone know if this is a known issue?  Planning to send the camera to Canon for repairs.