Why does my puppy sleep with a paw in her mouth?

We have a 9month old Husky/Labrador. She is still in her chewing stage and we have everything-we think-a rope, bones, Bimples-like a nylabone, and a kong to play with and encourage chewing. She's healthy-all shots-groomed, etc....We take her on 30 minute brisk walks daily, and throw a ball around for her in the yard. She's always full of energy! Sometimes when she lays down to relax on her side she'll have a paw in her mouth. It's cute but what causes that? We've had her since she was 4 months old, and she just recently started doing this. My wife thinks it may be like a pacifier or a security blanket. Any ideas? Thanks!