As you probably know, there is evidence for Ephesians being a circular letter, in which it would have been shared with Colossae and Laodicea and the other churches in that area. Perhaps someone at Colossae, (or somewhere else for that matter), who had both letters at some later date, compared them and inserted the phrase in the margin somewhere in the Colossian letter. Later on, someone might have thought it to be original to the text. Just some speculation.

Blessings!

__________________
Charles Plauger
Member/Grace Reformed Church
Oakland, MD