Since there are no in-game secrets, or at least not many, this section will also cover Tips/Tricks to gameplay.

Thanks Yuber8900 for these tips

Keep Williams
on the level where you must gain access to the weather station go to the side of the building to your right. chuck a Grenade under the vent and the door will be blown off allowing access. If you had gone in regularly Williams would have turned into a Thing this way you'll save many medikits for the levels ahead

Two Medics
In submersible beta where you must find Faraday don't hit the switches until you've met the medic downstairs bring him up to the top of the ramps and tell him to say. Hit the switches and go to find Faraday leave and on your way out the
medic won't have turned and you'll get two medics for the rest of the level.

These both work and I always use them makes things easier so you don't waste Medikits healing yourself which is useful for the first boss and making it through the submersibles

NPC in submersible Alpha
This one is frustrating and very very hard O.K. first you need Medikits, 2nd you need to have killed every single thing before the Room with the Hydraulic Override 3rd save the game 4th use the CCTV terminal where you can watch him die but as soon as you enter it leave it and run to the Override as fast as possible if your lucky and fast then he'll be alive so you can heal him if your to slow then he'll bleed out or get whacked by the walker when you've saved him check his name if you don't believe his name is NPCEntity