Your concrete is most likely still drying out. I'd say that's the source of the moisture.
Needs constant airflow.
If me, shed is up with 6 years.
DPC under it.
Using the airchamber with 2 or 3 years.
Without it, the bikes were dripping wet in winter especially frosty mornings.
Something to do with the dew point afaik but not that educated on that sort of thing.
You'd know better than me.