Long ago ?..

..( sorry to have to disagree .. I liked one of your posts on another thread...

)
No , I don't think it can be more than 3yrs. or so since Benjen went missing. (At the very maximum, four ) Arya was 8 when she left WF..Jon was 14..somewhere, more recently ( when he receives the engagement letter ?) I believe Jon thinks Arya would be about 11 or so ( maybe 12 at most ). When Jon meets Alys Karstark , she looks to be too old to be Arya.. She says she was 16 on her last nameday...Jon thinks she's ...
almost of an age with him.. again , allowing the most time possible to have elapsed , that would make Jon 18 ( or maybe as young as 17). But even if we take the oldest estimate for Jon , four years would not be "long ago" to the CoTF. ( I think it's more like 3 yrs, but I don't mind calling it 4 for the sake of argument.)
I do kind of want to tear my hair out when people say, Benjen wouldn't break his oath to the NW , though. Are we to think that Benjen is less intelligent than Jon ? If Jon can see that outworn convention has to be cast aside, and it's the spirit of the core tenets of the oath that count, I'm sure Benjen could as well...and wherever he is , why should we assume that his intention is
not to return to the watch ? Why should we assume he has
deserted ? Perhaps , on his ranging , one thing has led to another and he still has something important to do first.
Though I can see possibilities for the HM to be some other Northman, I think Benjen would best bring together the various hints and suggestions strewn throughout the novels . Winterfell and the Wall both built by Bran the builder..magic being used in the raising of the Wall ..Must always be a Stark in WF ..secret passages..Starks having a special relationship to the Wall , and being looked to for leadership by the Northerners .
So , Benjen left on his ranging to learn what was going on north of the wall and if the Others had really returned. We know the CoTF and BR have been guiding Bran. We know they have CH at their disposal, to aid people they see as important in the struggle against the Others. Why not Benjen ? If they see two Starks as key to their efforts , why not a third ?
Of course, GRRM may have another path entirely in mind for Benjen, but if he does place him in WF, there will be good reason for it, and I'd bet without tarnishing Benjen too much..
If he's been briefed by BR and the CoTF and returned to WF by way of the gate under the Nightfort, he could still have left the obsidian and horn at the Fist. If he travelled by some other route ( CoTF tunnels), CH could easily have left them.