Sadly, with many years experience as a householder in various properties, I fear it was, and still is, plain old condensation.
If you have no wall or ceiling vent in that room, or an opening window, it would be highly likely, so you can only mop it away, dry out the room, and keep the door shut to stop more moist air entering. Polystyrene liner could help, and then you'll find it will land on the walls, and so on...
Fran
December 2009
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