Mold/mildew smell

Two days ago, my husband and I came home and as soon as we walked in the door we smelled this damp, mildew smell. We hadn't had any problems with our house smelling before. It seemed to come out of no where. We cant find ANY moisture in the house and have looked everywhere.

Has anyone else had this happen? Any suggestions on what it could be? Obviously our biggest fear is mold since our baby girl is due in 7 weeks and we don't want to bring her home if there's a problem. But it seems odd that there's seemingly no source of the smell.