Labs like all gundogs have been bred to gain pleasure from holding things in their mouths, the hold is innate, the retrieve is taught by teaching recall. If she is a nervous dog, holding something in her mouth, may also bring her comfort, and it will certainly make her feel better. It sounds like she may be a little worried by you coming in, so goes into submissive behaviour. Some dogs do make a noise whilst holding, it is impossible to say without hearing the dog if it is pleasure or distress, or a mixture of both. make sure she has toys around so she can do her normal behaviour of holding. She probably needs confidence building if she is nervous, a local behaviourist or dog trainer would be able to help you with that.