More commonly than not it IS a crack in the wood. From the outside it may look like a little shallow crease if you press on it:
But inside the crack will reveal itself if you look inn there while pushing a bit on the outside:
My preference is to glue in a 5/8" wide )3/32" thick) mahogany reinforcement that spans the full distance between top and back lining. That will keep the crack from spreadong when it gets leaned on in normal use.