Just because something is dull and grey doesn't mean it loses its shape. The doorknobs/hamdles/pulls, surrounding bezel/frame and the slightgaps and depressions around the edges of any door would still give them away.
But it's all the same colour and texture. If they spent time closely examining the walls then I'd let them notice these things, but otherwise no.
Ever looked at a 3D model of a building that's doesn't have any colors or textures added? I have, thanks to 3D drafting programs I've worked with. Same color and texture doesn't mean same light level, and the raised edges of the bezel and knob, as well as the gaps around the edge, all create areas of highlight and shadow that make the door pretty instantly recognizable even as a grey-skinned outline.
I'm not saying they'd be able to tell the colors, textures, material (well, maybe a wood surface-layer from non-wood, but nothing else and not what's underneath), if its locked, or anything else, just that there's a door-shaped set of features protruding from the wall... well, unless there were no shadows to give depth to the object. Of course, there might not be; I haven't studied the Astral Projection stuff too closely. Still, that's the only reason I can think of that you wouldn't be able to tell a door in a casual look around a room.
I could understand requiring them to burn a Simple Action for "Study in Detail" in combat, but if they're not working in bullet-time (pun intended) it's just plain silly to actually make them declare a search to go "Hmmm, raised rectangle a couple inches thick with a crack along the inner edge and a knobbly bit about waist height alone one side of the panel in the middle. Wow, let me go study this in detail and see if I can figure out what this mysterious feature might be!!"