I Should Have Written It Down

A few weeks ago I was looking for something that I was sure I'd written down somewhere, and I found myself saying to myself, "I should have written it down." I did not mean that it would have been a good idea to write it down, and it's too bad that I didn't. I had failed to do something I should have done. When I tried to think about it more carefully, though, I wasn't sure what exactly I had meant. I didn't mean that I had done it because it was a good idea. What exactly is 'should' doing in the sentence?

I meant something like what we mean when someone asks us where the comb is, and we say it should be in the bathroom, fully expecting it to be there and ready to be a little upset upon finding out that someone had moved it. But what's different about this case is that there's no other person involved. Is it that I expect it to have been written down, and then I'll get mad at myself for not doing it if it turns out I didn't? I'm not sure at all what's going on here and how the components of the sentence contribute toward what I thought I was saying when I said it.