This guide will help you unlock the bunker with only half the notes required. You should know that you will need to be somewhat patient if you attempt to try this strategy.
Note #1: In one of the two buses
Note #2: Found in the area with the most houses, on either the table or one of the boarded up windows
Note #3: Underneath the Tower
Note #4: In one of the bunkers without a door
I recommend finding two notes that spawn relatively close to each other or close to the locked bunker (a bus is nearby it).
If you had to hide from Siren Head while heading to the bunker, then check the digits on the two notes you collected. Remember what placement the digits have.
In order to unlock the bunker with two notes, this step is crucial. Interact with the bunker door to get the digit input screen. Once you have that, enter the digits you have already and fill in the blanks with 0, and keep going up until the door unlocks.
Example: You have collected notes #2 and #4. The digit for note #2 is 2, and the digit for note #4 is 6.
You would input the code like this:
If that fails, keep going up, like so.
Once you have done digits 0-9, move up the first number to the next digit:
Lets say the door code is 3246. You would reach this code in 45 attempts, which goes quickly depending on how fast of a typer you are.
Granted, it is easier to enter the code when you have all four notes. But, Siren head becomes more aggressive with the more items you collect. By collecting only half of the notes you need, Siren Head will be less aggressive and is less likely to teleport directly in front of you.
This allows you to get the secret ending involving the matches with only collecting 4 items instead of 6-7 items, thus making getting across the map from the bunker to the gas station easier since Siren Head will be less aggressive.
I’ve done this strategy before, and it helped me get the secret ending.