In the spawn room of Mauer Der Toten, there is a painting that produces lots of bubbles when explosions go off on it. It also produces a smaller amount of bubbles if shot with a weapon dealing explosive damage. Finally, any molotovs thrown at it disappear into the painting.

This has been a known thing for a while, and noone seems to know anything else. It is unknown wether or not there is more to this, or if its just a glitch, or whatever.

Now for the interesting bit:

On one of my recent matches, after messing around with the painting for a bit(I threw some frags at it and shot it with a normal gun a lot), I went to the mystery box for the first time(early rounds, hadn't turned on power yet). The box was in the Staminup location, which by the way, happens to have an identical painting over it(more on this later). The crazy part is that the box had a lot of electrical energy coming out of it. I don't remember if there bubbles or not. I spun the box and nothing interesting happened, except the electrical storm effect went away.

Has this ever happened to anyone else?

Another note about the painting is that there are lots of clones of that painting in the Staminup building. I'm not sure if there are more anywhere else. None of them produce bubbles or seem to do anything special. The most notable two I saw were the one directly over the Staminup box location, and one that has a Gallo wall buy pointing directly at it. Using the gallo seemed to do nothing, however.

Finally, I did notice that when the box changes location due to spinning a bunny, the box flying away animation does have a lot of electrical energy, like what I saw when messing around with the painting. I'm not sure if it was exactly the same or not, but it definitely does make me wonder if I in fact did not make progress on a side easter egg, but rather just saw a rare graphical glitch making that effect appear erroneously.