In some of the mercenary camp areas you'll find pallets of white drug packages stacked up. Take a closer look at them and you'll see they're stamped with a scorpion logo, which resembles the one used heavily in Just Cause 2.
This symbol is also used by Mexican cartels when smuggling drugs, but with a similar tropical island setting and free roaming ability in both games this could be more than just coincidence.
You may have grabbed hundreds of loot items and flogged them off without a second thought, but if you take the time to read the descriptions there are a whole load of references and amusing details to discover. For example, the blurb for Beautiful Seashells says that "it won't be hard to find a buyer" for them, which could well be a reference to Bond Girl Honey Rider in Dr. No who makes a living from collecting and selling seashells.
There are plenty of items that will raise an eyebrow such as a pair of pink handcuffs, or a photo of a naked dwarf who's laughing and pointing at the camera without using his fingers. You may also stumble upon a rusty trombone, a corroded instrument that actually refers to a dubious sexual practice, which we wouldn't suggest Googling if you don't already know what it is!
This isn't an Easter egg as such, but it is an interesting part of the game that can be easily missed if you don't notice it. After completing missions where you rescue one of your friends and end up back in the cave under Dr Earnhardt's house, you'll find a table to one side with a bowl of red pills and an "EAT ME!" sign - another reference to Alice in Wonderland following the quote during the opening sequence.
Interacting with this when the option is available makes you take a pill and experience flashbacks of events leading up to your capture on the island. Here you can explore a nightclub, buy drinks for your buddies at the bar and even bust some moves on the dance floor - everyone loves doing The Robot, right?