The lack of a secret counter and more secret areas was a bit (but just a 'lil bit) of a letdown for me, although understandable given the game's less 'arcadey' nature and more focus on ambiance and horror. Still, way better than 2016's Doom (and by extension Doom Eternal) in my opinion.
As for the secrets, I remember this one in one of the Alpha Labs levels, forgot which one, in which you have to shoot the floor and a tile comes loose giving access to a hidden area. There's a weird radio feedback as you go through a small tunnel or something, that leads to a section with an imp, an exploding barrel and a few goodies. I'm still intrigued by what that strange radio transmission could mean, though.