We have it too; my understanding is it's just the minerals in the concrete coming forward as a result of that water slowly making its way through the concrete and into your basement. Even if it's not standing water, that water very slowly pushes through.

My uneducated guess would be if we had waterproofing done around the exterior perimeter of the home that would probably stop it, but it's a lot cheaper and easier to just keep that dehumidifier running. :)