Rust on bottom of opening to bedrooms

[Jennifer MacDonald]

Anyone know why rust type residue would be on the bottom of the opening to the bedrooms (pic attached)?  There is no actual door in this location just a cut out in the wall that leads to the bedrooms.   You can see it at the very bottom of the opening at the baseboard level.  I saw this in person and touched it-feels like rust.  Maybe a metal band about the opening to texture the wall? 

Any help appreciated!