You missed 3 secrets in Hazard Disposal.
The first missed secret. Resident evil 1 lab in E6M2. The door is blocked by a lattice with the inscription <SECURE AREA> (you can go through it) before the last secret in the video.
The second missed secret. E6M2. Not far from the elevator there is a room with a green door where there is one enemy. Behind the grid, there are objects in the acid. There is a passage to the right of the panel on the wall (click use).
The third missed secret. Before the final level and the blood pool, there is a green door at the entrance to the corridor. An object is a visible behind this door. There is a blue cabinet near the door. Blow it up.
I hope I helped someone. :D

Yeah in E4M4 you can go back after the ambush, same with E5M3: after the fight in the office you can go back right before the start of the level by opening the gate on the right and then climbing the ladder back up.

In E4M4, there's a switch after you clear the ambush that allows you to go back.

E5M1 There is a REAL secret-not-a-secret, if you use the jumping boots you get in the outside area near the end of the level, use them to jump up to where the waterfall is, then you have access to the perimeter around the 2 massive valleys. I won't say anymore but see if you can find it, there is a funny message inside as well.

Gotta say, it's a surprisingly long game with big levels. Thank you for the guide.

yo you forgot yet another secret achiv in 3-rd zone in corporate chaos I think

ah, need to BLOW up this tube to open it, gotcha...

yeap, that secret is sealed of for me too ;/

hey in e5m2 the secret to get the extra damage power up is locked off how do i open it up?