Yes, CONDENSATION 
I have a thermometer and humidity monitor in the garage which I find really useful especially when our MX5 is stored over winter. I came across a recommendation for non electric dehumidifiers. Designed primarily for window sills & similar places, I thought one might be useful to put inside the MX5 cabin. I placed it in a plastic food storage container because I had no idea how much water might collect and obviously I didn’t want it to overflow into the car. I check it most days and depending on the weather conditions it seems to collect about half an inch of water every couple of days which is easy to pour away. I would add that the car is in a heated insulated garage which is integral to the house. I would imagine in an unheated stand-alone garage, the amount of water collected would be significantly more. It cost £7.50 from B & Q and the doughnut shaped gizmos that do the work are available as replacements separately. Blyss Curve Moisture trap
I use the microwaveable bags one in the boot one in the cabin work well and cut down on the plastic waste too
So do they absorb moisture & then you put them in the microwave to evaporate the water? How often do you have to do it?
The Blyss unit is reusable, just the moisture collecting doughnut needs replacing periodically.
Depending on the conditions but, I check them weekly there’s a coloured blue indicator that changes to pink when they need drying out
My MX is in an unheated attached garage with no insulation.
I open the garage doors regularly and let the air do its drying job.
To keep the moisture at bay in the cockpit, I take it for a well deserved winter drive at least once a week, providing the roads haven’t been salted.
Also helps take the edge off short winter days!
The problem is exacerbated by a period of cold temperatures followed by an increase in temperature. The moisture in the air condenses on cold surfaces and they appear to “sweat”. The problem is that the moisture is ever present but not always noticeable, one reason why I keep a humidity meter in the garage. In our previous unheated garage it was a constant battle against the elements keeping my motorcycles dry over winter. Any covers had to be removed on dry days to allow the air to circulate otherwise they simply trapped the moisture on the bikes.
Mains electric dehumidifiers are not too expensive to buy and economical to run, certainly cheaper than a fan or oil radiator in an uninsulated garage.
Which is what happened to me a few times when the MX is in the garage and temperature dramatically increases after a cold spell…
Ugh! Gives me the collywobbles! Lovely looking car otherwise, I do like a white car with black wheels 
Not many of them left…!!
Which model & year NC?
2011 Sport Black.
Owned it for 11 years now👍🏻