On course or On a course?

There seems to be some confusion about the correct term to use when informing people that you are away from the office. I often receive out-of-office e-mails that say "I'm on course". On course for what? On course for reaching your target? On course for getting the ship to its destination? On course for what? "On Course" means going in the intended direction.

So isn't it a little strange to say "Mary is on course today"? The correct term should be "... on a course".

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