Smell from corner of conservatory

[Misterbilliam]

I've moved into a property and been here 6 months and in the last 6 weeks noticed a smell coming from a corner of the conservatory. Lifted the floor up which has a concrete base, beams resting on top with mdf on top of that then tiled. Strangely when I lifted the floor in the corner I can see that there's a channel (gap of about 6 inches) where the concrete base has been built around the old step. I'm wondering why this would be? and whether the smell is musty air from this space coming up through the gaps near the skirting. The smell comes and goes so I'm thinking it could be down to wind direction. There's no air bricks visible on the dwarf wall of the conservatory. Any ideas??

 

[theprinceofdarkness]

Have you peered underneath with your wifes compact mirror and a torch. There could be a manhole somewhere under the floor, now covered. Where do your drains run?
Frank