I think that may be from a tree. I had issues with a tree in front of my house, I would park the car near, but not under, a deciduous tree for a day or two. It was sunny weather with no rain or clouds at all, and I would have a slightly sticky film in a droplet pattern on the tree only side of the car much like in your photo. I got to it right away with rapid remove and it all came off, but soap would not work. This was all from my tree weeping from the leaves as a thin sappy like stuff, my driver side window squeaked for the next few years whenever you rolled it up!
Now it also looks like heat from the engine may have baked it right into the matte finish, so it may never some off if I am correct in my theory.