when dogs chew its can be because they are feeling anxouis or stressed, chewing releases endorphins which help to calm and sooth a dog down, a dog will chew to help itself relax the same as a dog will eat grass, when its got a upset stomach, to make it sick.
What I do with my dog is to fill a empty lamb shank bone (quite large) with a bit of meat in the middle then her dog meal biscuits followed by normal dog meat at each end to pack it all in. I completey blank my dog for about ten minutes before I leave (if you make a fuss about leaving your dog then your dog will think that they is something to be worried about) then just put down her bone and walk out without saying a thing.
I find that because she wants the meat she will spend a good 20 minutes or so chewing on the bone (as long as the bone is well packed) that by the time she has relised I have gone she is already relaxed from chewing and settles down with no problems.

Pets at home do Lamb shank bones which are perfect for this little trick.