[John Norstad, the creator of the definitive free Mac antivirus app,] it’s a practical joke in the ROM code in some third-party keyboards [that if you didn’t type anything for a while they would themselves type ”welcome datacomp”.]. The only solution is to get your bad keyboard replaced.
My guess is that this wasn’t a practical joke as much as some sort of firmware test gone awry, but regardless, “welcome datacomp” managed to appear in numerous publications, as Haller’s post details.