Are there some Japanese greetings like the English "hello"? Most greetings seems to be tied up temporally or by another specific situation ([もしもし] for phones, [こんにちは] during day time). If I left a note behind for someone else I would have no idea how to greet them in the note. In English at least it's easy as "hello" always works. "Hello X, by the time you read this..." or "Hello, if you found this then..." are two examples. I don't see any of these being replace with other greetings such as "good day", "welcome", "nice to meet you", etc.
So what are some of the more generic ways Japanese people greet each other? If you were to leave a note without any idea of when or perhaps who might read it, how would you greet them?