The messages that I've seen direct-messaging someone or replying to an un-related post were clearly spam.
If you start the 'report and block' process on a message, the software (on my server anyway) shows the account's other recent posts, and asks if they illustrate a pattern of spam. Usually all the account posts is begging messages, across every server it can reach.
If it's not a person you've known for some time, block and optionally report; assume they're scammers.
