I have used both the P45+ and IQ160 in light misting rain several times. Never had any issues, but I also had a towel to wipe the moisture off. The DF+ may be another problem, but my DF has been through quite a bit and still works fine. I would be more concerned about the DF+ in heavy rain conditions than the back.
I also carry a clear bag in my case just in case to allow me to cover the camera/back now.
What's the difference between the DF and the DF+ that would make you more concerned about it?