Has it been out in the big storms / parked under a tree ? The windy / wet weather can drive water in through quite small gaps. I would check the windscreen drain under the bonnet and see if it's blocked with leaves / pine needles / other gunk. I am aware of similar problems with other cars due to drain being blocked so water gets into the car. Is there anywhere which is particularly wet inside the car ? I would also check to see if the external rubber seals are good on the windows and if the drain holes are clear in the doors (if the car has them). Likewise around the tailgate.
Plan A would be to dry the car out, ideally with a dehumidifier, and then see if it returns and also this might identify where the water is coming from.
I had an old mini back in the day and water used to come in through front footwells due to rust issues.