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
- Most importantly: 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.