I think you are right.
You always get set as the belly takes some compression.
But it can be diguised by building in reflex when you start with bending, heat treatment, laying up or splicing in backset.
The Osage bow I made had about 1" of backset spliced in and ended up with about 1" of visible set (but that's 2" in reality).Similar thing with the longbow I use, I heated in about 2-3" of reflex and now it's about straight.
Any spring will take some set, the valve springs in your car engine take some set, as do the springs in the suspension.
Mind you there's now't wrong with a bit of set. Yesterday I picked up my little 40# Hazel bow which has tons of set, I hadn't used it for ages and I was surprised by the cast and shot a great group, smooth as silk.
A bit of set shows the wood is doing some work.
Oh, and as for pictures, you'd need before and after tillering shots to really see if there is no set.
Del