Help me locate this smell!!

Albermarle

If you have a suspended floor ( typically wooden floorboards resting on wooden joists). Then the space underneath needs to be ventilated with moving fresh air. Otherwise damp/moisture will build up and can cause the wood to rot.
So airbricks are fitted at different points around the house, so air can flow though the space. Ideally there should be 4?, but often there are less.
You should check them occasionally to see they are not blocked. Just poke a stick/screwdriver through the holes.
One issue is that if there has been an extension with a new concrete floor, this can block off the airflow, if this is not taken into account when laying the floor. 
Obviously I  can not take into account crazy neighbours actually poking things into your airbricks, as it sounds too bizarre !

Lorian

Does it have tones of sweetness or does it smell earthy (wet dog)?

If it comes and goes do you have a mop? If so smell the head about a week after you have used it.