Did you guess again and again until they opened or were the locker codes somewhere in the demo that I missed?

safe code is
9 left
15 right
7 left

you get 2 extra inventory spaces via a hip pack

@HI_BLADE83 Where do you found it?

the only thing how you get the code ? pls show me

@Sixth_Doctor the Shotgun is located in the Locker room- you get there by "story" - where you jump the window, on the table after you entered there door where you would locate the licker in the original RE2 - you go through and find the weapon locker card, after that you climb through the window, a metal door opens the weapon locker where you can enter key codes, if you make your path through the room you get to the locker wich requires the card, there you get the shotgun and some shells.

I found them also alone)

I found one set of 4 shotgun shells, but not the shotgun, so I just left them where they were when I played the demo. Zambinos went down after a few well-placed headshots anyway.

Yes, there're no hints to the codes anywhere in the demo, so i figured they're gotta be names or abbreviations related to the series.
And since there're very few vowels on the notches, it doesn't leave you with a lot of options for that.
I ended up figuring each one out in under a minute.

So people just tried every combo? I mean codes are not shown anywhere in the demo right?

@Sixth_Doctor, it's shotgunshells for 2F and Mag-Rounds for 3F (No Magnum available in Demo ofc)