I have a book called "Guide to North Carolina Highway Historical Markers."It lists 19 markers in Warren County.None have anything to do with an Allen Plantation or anyone named Allen. The book was published in 2001.
If there is some kind of marker, it may be a private one or one that was put up by a local historical society instead of by the state.You might want to check with the county historical or genealogical society.They might have information.
Where did your information come from?Could it be that the historical marker is one of the 19 and was used as a means of specifying location but not directly related?If you do find out the location, I would love to know.