If I found something I thought was a Death Note I would spend a long, long time meditating on the question of how and in what way I'd gone insane.
Actually, I think most of the measure of people having Death Notes is... in Death Note itself. Thus, if I had a Death Note, I would logically conclude that the most likely explanation is that I myself am a character in Death Note. Not in the original manga, of course, as I read that and I know I wasn't in it, but likely in some spin-off. I could easily see myself as a character in some sort of Death Note video game/simulation.
I am on the fence about the Simulation Argument, but even so, this is exactly the kind of thing which is strong evidence that I am a...
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.