A leak or moisture getting into the cabin or boot area. As, I think, Geoffers suggested, it could be off wet or snowy footwear leaving carpet or mats damp. That moisture can condense on to the screen/glass in the right conditions.
Presuming you are not losing coolant through the heater matrix leaking on to the carpet to the left and up from the clutch pedal... or the matching situation in the front passenger footwell.
Presuming, too, a window/sunroof hasn't been left inadvertently open (just a little even) during rain in the last month, nevertheless it is very likely to be rain water ingress somewhere...
If you know you haven't been much getting into your car with dripping soles on your footwear... and yet when your car has been parked all day or night and the inside glass appears to be much more condensed/wet than seems to be the case for nearby cars (in your work carpark at the end of the day, say), it's almost certain you've got a rainwater leak... somewhere.
The exact site of the leak may not be easily obvious. Like an Escort where, luckily, I had a rough idea where water was getting into the hatch but I had to strip a little interior trim to see exactly where.
Another car where it was sneaking past the 'sealing' gasket of a rear light lens. Only found by removing boot-lining interior trim.
Yet another where between the carpets and the floorpan was found to be saturated but deceptively little, if any, moisture was noticeable topside. Deceptive because the carpet undersides were plasticised/water proof so little, if any, water could get up from the underside. The cause was bad seal for the bottom of the plastic liner inside the doortrims/cards; the water was tracking under the carpets' retainer plastic trim immediately adjacent to the door-shuts to then track under the carpets. I'd kidded myself for too long that the unusual amount of moisture (wet!) inside all screens and glass must be normal in this newly acquired car. And instead of dealing with the cause I later found I had simply been masking the symptoms through summer till the car started to obviously stink in the wet autumn. Left too long, it was a pain to dry out.
Each time someone playing a hosepipe on the outside of the car, while I was inside systematically looking for the leak found it. A torch can be useful... specs, too, a must if you use 'em... as a water drip can be surprisingly difficult to see. It can take patience to find but the 'fix' is relatively easy. Got to get at the cause of the excessive condensation and not fiddle with symptoms to mask it... as I found with at least one painful experience.
