Do spammers actually make subject lines such as “[SPAM]”? Or is this added by some filter?

I have a Gmail account. Today, I had received a spam message whose subject line was actually “[SPAM]”. Is this some new kind of “trick” by spammers, or was the subject line determined to be 100% sure SPAM and thus marked as such by Gmail or “something else”? And if so, what is “something else”?

Technical detail: I don’t have any post-processing on my side; I fetch the e-mails directly from Gmail via IMAP.