Prominent nonsecrets in this game
A nonsecret is a hidden item that doesn't classify as an actual secret. There are a lot of them. If you know of any not listed, please share them. I know of more than these but I would have to play all chapters again to get a feel for exactly where they are, but ones I remember are as follows:
In E3M1(Raw Meat) there is a destinctive grill which exudes steam. Standing on this grill does inflict small damage. Roughly in the middle of this grill, if you try to "open" the wall, a small patch of wall opens revealing the jetpack. This allows you to achieve the rocket launcher early in E3M2(Bank Roll) In this same level there are a system of secret teleporters(one can be found in the nightclub, one in the room that has all the japanese tables and so on. Check the walls and the poster in the night club to the right of the stage, the dancing girl on the stage, and the microphone.
In E1M1, if you are at the bridge which is connected to the level exit, and use the jetpack to fly directly up, you will eventually come across an enemy auto turret, next to a compartment with shotgun shells and a fresh jetpack.
I forget which M it is, but in the secret level Tier Drops in chapter 3, you should start in a hallway like area. There is an opening in these hallways which reveals several structures, usually populated by enemies and items. In the absolute corners of these hallways, there are doors(which I believe open), but, less obviously, there are electric panels beside these doors which open aswell revealing items.
Nonsecrets tend to be most common on levels with no official secrets, and also secret levels. The entire Duke Carribean: life's a beach chapter is PACKED with nonsecrets around almost every corner. One of the most important ones being that one of the slot machines on the only secret level of the expansion opens revealing atomic health and I believe a rocket launcher.
On The level Mr Splashy's of the same expansion, a soda machine can be pressed causing a door to open revealing "hot sauce"(roids.)
Know of any of these not listed? Again, please add them, let's try to start a list here.