I think the thermal imaging idea is a good one. I see now in the last photos the corners have what “appears” (yeah, everyone hates that word) to have open seams. These must be like you say from a couple generations ago vinyl windows as any today worth a flip are gonna have 'welded" seams at all the corners and miter joints. That is exactly why I replaced (still in the process) all my old aluminum windows with the new vinyl types.
We get rain here that is sideways coming at the side of the house and you need all the protection you can get to keep out water. I saw water shooting in around the garage door windows with tight rubber gaskets during Hurricane Dennis and it was spraying all the way to the far end of the garage. Made a believer out of me.