(Exact same thing also posted on Steam)

So is there anyone who can hit that 100% secret completion with any reliability?
There are some levels where I get 0% almost every time and for the life of me I can't figure out what I'm supposed to do.

Here's the breakdown, to the best of my current knowledge:

1-1: shootable switches high on the walls (1-2, maybe 3?), opening rooms with health or armor. Sometimes there is a power panel on the floor below them. Don't always add up to 100%, so I'm missing something.
1-2: The Luftenstein machine counts as a secret. Unsure if it still does if it doesn't spawn because you don't have the token.
There is at least one hidden room with a special weapon on this level. It can be found by blowing up walls with barrels, and as far as I can tell ONLY barrels (machinegun grenades don't work for example). No idea how to determine which walls can be blown up.
1-3: No freakin idea. Maybe there are shootable switches but the level is so dark so I never see them? (still, no panels on the floor, ever...)

2-1: shootable switches on the walls, these are blue and pretty easy to see, but again I'm not usually getting 100% (do the STRAFE letters count because I don't usually get them)
2-2: the bunnyhop course might count as a secret, other than that I don't know. Getting the DS battery from the annoying floating robot might count as a secret too?
2-3: the DS mining game probably counts, but I haven't confirmed that. Other than that, again, I found nothing.

Burbs: sometimes there are shootable switches on the walls, which look like the ones in Icarus
I imagine the extermination game also counts. Still, there should be more. I usually get 0% on 3-2 and 3-3.

Athena: shootable switches like before, in all levels. I can't always make them add up to 100% either. The flowervase game probably counts.

Occasionally I get some extra secret% at the end of a level and I have absolutely no idea what I did (happens on 1-2, 2-2, 3-2).

Please add to this if you have more info. Some of the levels are driving me nuts.

burbs: shoot the presents on the beginning of the first level till you get the cartridge...then when you later meet the shop, attack his owner and get the controller out of his office...use both on the console in the last burb level and voila:

an really really cool secret

The secrets in Strafe are really hard to find. Honestly, I never find any secrets in the Black Canyon Zone. I always get 0%.

I have one addition. In the Icarus Zone there is one special room. It´s a bit tricky to describe. Basically it is a corridor, you can enter through a moving wall. It connects two walls of a much bigger room or serves as passage to other rooms. You can also enter this corridor through a hole in the ceiling, and sometimes it gets counted as secret. Though I couldn´t find a rule when it is counted and when not.

Some other things, that are puzzling me: Every world in the game has three levels. But on the maps at the end of every level you can see more sections than the ones that you visit. Could these be secret levels?

In level 2-2 there is always a closed door, that looks like a second exit. Is it possible to open it? I tried everything, but can´t find a solution.

I found a new one! Not sure whether I should spoiler it or not, because it´s nothing you can trigger intentionally. Sooner or later you will find it incidentally. It is in the Black Canyon.
If somebody wants to know, just ask :)

Yes, it is. But it has nothing to do with the RPG mode. The secret was in the game from the beginning, before the addition of game modes.

Yeah I realised, I posted in the heat of the moment as I was in all kinds of awe at weird stuff happening for the first time for me. You really have to work for some cheevos after I did some research. Didn't ruin the game for me, just gave me more understanding of what I'd be able to do in a playthrough, which is cool AF if you ask me :)
I discovered one of the guns, and the glitch shop in a secret playthrough and suddenly it was the best playthrough EVERR!