In mission 3 in sb2 there is a door where ellis form l4d2 says a story
Many doors or boxes play a sound when pressing on them ( for example mission 2 in sb2 )
Ofc. Portal refrence and assian guy lauging
(4)

There was a reference to metal slug in sb2 level 6

shooting the randomly parked BRDM outside the friendly base with a TOW yeets its turret as well as a random person inside it

Nocliping through a door just outside the office in Horizon Town takes you to a door with a sign that says there's a private party going on in there.

in mission two we can hear the AUGHHHH AUGGHHHH sound effect when the radio guy talk to you

in mission 2 there is a ref to a guy named "maverik" that crashed a jet, probably a ref to top gun

there is a room in mission 2 with a "danger explosive" sign and a guy screaming inside of it, if you wait you will hear a big boom

in mission 2 there is a breaker behind the base, press e on it to hear a fart noise

the bomb that blows up the tank has a little little baloon to slow it down

in mission 2 you can use the missile launcher to blow up a apc

in mission 4 there is obviously a ref to portal and a door you can't open without a keycard and in mission five there is a snake plush next to the lake and the guy in the helicopter makes funny screams

in mission 6 there is door you can interact with; you will hear the aUHHGGGH sound effect once again, there is also a room owned by chad with metal slug running and a room with metal gear for itchi, level 6 has the apc , the breaker and the apc easter eggs that we already could see in mission two

there is an explaination about the music you hear while fighting too in itchi's room

in mission 7 there is blocked door that you can also interact with that has the aUUUUGHHH sound effect, you can find drugs in the enemie's base and the radio guy will say something about a girl named "sniper wolf" eating it like candy. in mission eight there is a cinder block named brock

in the credit map brock apear next to you (what a nice man), and a car drive directly into the construction site like it was nothing. this was my final easter egg report, there is more "easter eggs" like the metal pipe sound and the weird laugh of the people in the helicopters i didn't mention because its not really a secret. buh bye