I think there's even an extra layer of nuance here as well. At the risk of sounding prescriptivist, "I don't know what this is" is a statement, not a question;

I think there's even an extra layer of nuance here as well. At the risk of sounding prescriptivist, "I don't know what this is" is a statement, not a question; appending a question mark to it changes its tone, and seems to me at least to give it an implicit, almost sarcastic question; "I don't know what this is. Do you expect me to?". It puts the responsibility for the statement, the responsibility for the confusion, on the presenter of the information, not the receiver. It's less innocent than pure confusion; has a little more sneer to it, or at the very least, a little more snark.