It appears (in Windows 10 with anniv update) that the F3 feature to "Find Next" within Notes and Documents is no longer working. The find next button is working but not if you just press F3.
F3, Ctrl+F3, Ctrl+F all appear to work in Windows 10 Home Anniversary. I've also tried Pre Anniversary Update.
I'm not able to recreate this issue.
My suggestion would be, see if you have any other HotKey or system process taking control of the F3 key. Some laptops do this.