In the college, there is a board where the developers have signed their names.
While buying guns, it is possible to hear the warning “careful, you’ll shoot your eye out,” a reference to the movie A Christmas Story.
At one point, O’Brian suggests that the infection could be intentionally targeting certain genes, which leads Deacon to wonder if he means it was terrorism. Later on in the game, you can find a document called “Classified Virus Research” in the research facility. The document references botanical samples from KemSynth Botanicals.
These are both references to the Syphon Filter series, a previous series made by the same developer as Days Gone. This has led some fans to believe Days Gone is actually part of the Syphon Filter universe.
The Stun Gun from Syphon Filter is also an unlockable weapon.
One of the trophies requires you to collect 989 freaker ears. If that seems like an unusual amount, it’s because it’s a nod to the publisher 989 Studios. That was the publisher behind the original Syphon Filter games, which were made by the same developer as Days Gone.