There are 2 drain holes at the scissor jack points, both front and back. It's easier to see how the front ones work, if you pop the hood and follow the drain trough, there is a hole that goes into the fenders, dirt, leaves and stuff collect and trap moisture in there, which then rots it from the inside out.
As far as I've been able to find, the rear drain holes for the soft-top rain rails, also get full of crap but overflows into the rear sills.
In both cases it eats it from the inside, so, by the time you can see it, its too late...
This video shows the problem with the front, not sure how good his fix was.
https://youtu.be/_ZCOl3U4uGUThis video shows how to inspect the rear.
https://youtu.be/frP_w7oRavoThe first 10-15 mins of this one goes over all the places our cars tend to rust.
https://youtu.be/PHrR9j7gNjYI've just gone through the process to find a clean NB, so I can tell you that you don't need to live near a beach to get rust in these cars... The more leaf litter and rain you have to deal with, the high chances of finding it under the carpet near the fuel pump, the boot is a common one as no one thinks to replace the rubber seals and check on the bottom of the doors....