It is better to check for physical causes first, as you are doing. Then medical causes. Odd smells often happen before migraines, although those tend to be really bad smells. And baby smells would be one of the worst to me, anyway. So if it is a spirit, try asking yes or no questions and see if the smell gets stronger (yes) or weaker (no) or stays the same (maybe). One of my first questions might me "Are you a Spirit?".