In the first map, there is a developer/contributor secret room found inside the hotel next towards the forklift event. Before patch October 2nd, this place was usually inaccessible, but now accessible seemingly out of the blue. However reading the patch notes it seems to hint at something, a developer losing their key. Currently I've found 2 out of the many other spots in the first level, one being towards the left of the saferoom exit next towards a door, and one travelling throughout the bar in front of the survivors and going left into the backside to find a truck. Apart from that, I am not too sure if more spots exist, although it's highly worth it to check for the room, because laser sights and mostly all the tier 2 weapons can spawn.

As hinted by the patch notes on the third map they added cola as an additional patch, but with no insight on what it does, it kind of makes people question what does it do. Remember how many people wanted to save Mike inside of this campaign? Well now you can by the power of cola of course.

If you bring the Cola all the way towards the safehouse towards level 4, Mike will join the survivors, you can even bring him as far as the finale and have him escape with the survivors. This is a highly unique and interesting easter egg and one that has a very special reward, unlike most custom maps that probably just reward the player with m60's and lasers.

Although I do wish I could ask the developers, are you able to add collision for the cola so they don't get stuck under tables/vehicles etc. Unlike the gnome and unlike Dead center's Cola event, this can get stuck under some of the most smallest gaps and it can be a hassle to get the cola out if you really want to do the easter egg.

Anyways those are newest easter eggs I've found, currently haven't found anymore, but I might update if something interesting pops up, and no I wont bother updating the old ones considering there are YouTube videos out there for that, this is just an updated one.