I’m mystified.
I’ve got a 2011 Ford Fiesta and just before Christmas noticed the front passenger seat was wet. Asked the Mrs if she’s spilt anything and to have a look in the morning as I wasn’t sure if was my imagination, and she confirmed the damp and that the back seat was wet too. The kiddy seats were dry though.
We managed to dry the seats out but after Christmas noticed the seats are wet again.
Google suggests some poorly installed vents behind the rear bumper allows rain water in but I’ve pulled the drain plugs underneath and these are bone dry. No hint of moisture.
No moisture under the rear seat, carpets, seatbelts, roof liner, window seals. The passenger seat fabric is not even in contact with anything, so where is the damp coming from??!
It may or may not be relevant but when we dried the seats out, we noticed white streaks on the fabric.

Checked Door or boot seals?

Bit of a mystery, is it possible /safe to leave a couple of windows semi open to let it air out?

Could the streaks be mold? Would take a while to get moldy if it’s just recently got damp though …

Are we sure the Mr’s or the kids aren’t spilling drinks?

Sunroof ? Other points, window coming slightly open – rear windows, are they wind-up ? My son’s had wet seats a couple of times in last 6 months when friends have left the rear window’s slightly open on his car.

Check the panel with the windscreen wipers. Sometimes these have drain tubes that become blocked

My parents focus leaked gallons of water in through the vents in the rear bumper, its taking months to dry out and everything was sodden.

My focus leaks through the roof rack mounts if i dont put the seals back on.

It can be hard to track down as its constantly evaporating and condensing somewhere else.

It can also be the air conditioning condenser drain, if thats blocked then it overflows through the front vents.

White streaks are just watermarks, just dirt/salts getting left behind as moisture evaporates.

Does it have a roof rack?
To fit them you have to remove sealer to reveal the thread and if too much is removed it causes leaks.
They stopped making the fiesta with bolt on roof bars around 2012 for this reason the newer models hook around the panels

Had this with a 2009 fiesta was the seam sealant under the black trim in the roof rails. Removed the roof trim and you could see lots of small gaps in the sealant that were sealed by the roof strip glue. Cleaned it up applied fresh sealant and refitted the roof strips.

What parkesie said