Owning your own home

bit of an odd one but seeking advice from strangers on the internet before calling on professionals (obviously). we moved into a maisonette which was built ~3-4 yrs ago, 4 flats and 1 other maisonette within the building. recently we got a chippy friend of ours to put a door in under our stairs for a bit of extra storage pictured below (we assume original developer didnt do this because we already have a very large "under stairs" storage a bit further along the hall and there is a metal support beam/cat scratch post in the way of getting in a proper big door)..... anyway with this installed we have started storing misc crap in there, but it just smells quite dusty/stale inside. sometimes when i walk past the closed door i get a bit of a whiff of this slightly odd smell... how can i remedy this? mrs swede says i am way overthinking it but it is getting on my nerves. it doesn't smell damp, nor do any of the walls inside look/feel damp. infact it is very warm in there. It is also right in the centre of the whole building so no real points of entry for water.... does it need a vent to be installed? does it need some of treating (plaster/paint) inside? could i just seal up the door a bit better and let the smell exist inside....? is the concrete still letting some moisture out and giving off a bit of a smell?

TL;DR have opened up a small area under our stairs in the centre of the building. a slight stale air/dusty building site smell emits from it... how do i stop it?

door opens up to extend to the right inside under the concrete stairs;

view from the front

directly to the right as you enter

further in (tallest part). the other side of this stud wall is our other under stairs storage

"ceiling" which are the concrete stairs

our other under stairs storage. the inside wall to the left backs on to this other new under stairs area/cave

thoughts?