I would suggest that these kinds of comments should always be phrased like the following:

> Your question is off-topic here, but on-topic for *xyz.SE*. However, please do not re-post on the other site, your question will be migrated there automatically.

> I voted to migrate your question to *xyz.SE*. It should be migrated shortly.

This will however need one of the following:

- A possible migration path for close voters
- The person writing the comment *flagging* the question for moderator attention to have it migrated
- Confidence that the question is really off-topic here and on-topic on the other site.

This means for every actively moderating community member that they should always

- flag a question where they drop such a comment on, *or* 
- vote to close appropriately, if and only if they are confident about what they're doing.

Comments without further explanation for the OP should IMO be deleted to avoid confusion. Eventually, a question will find its way to the site it fits best on.