Yes, as Eliot said, it's not a complete sentence. Many people write like this in emails, especially in the beginning of the message. To make a complete sentence, you can say: "THIS IS just a heads up..."

It's not a complete sentence. It's part of a sentence. The part of the sentence is perfectly correct though.

This is an "alert" message. The part of the sentence that is missing is "understood". Some other sentences that seem incomplete are: "Come here". The "You" is understood in this command...."You come here". "Just wait a minute" means "You can just wait a minute", "just a comment on your situation..." means "I have a comment on your situation" but the missing part is understood. This would be a casual way of speaking, of course.