I’m at a bit of a loss to attempt to explain this, but I have some ideas.
First is that it’s a leftover bunch of layers of a failed or partially-failed print that stuck to the tank surface, and eventually curled up and peeled off the tank, presenting itself how you found it later.
If it wasn’t that, I think there’s almost no way that the resin could have cured in that pattern with light coming through the black polystyrene tank cover, or light coming through the cardboard.
However, the pattern of the cured ‘alien carcass’ looks a little bit like the spacing between wires in typical wire shelving, and it’s a lot easier for me to imagine that shape curing against the bottom of the tank than floating on top of the resin, so my hunch is that maybe you left the tank covered, but sitting on a wire shelf for a while, and light from some source bounced up off the ground, through the shelf, and into the tank that way, curing the resin that wasn’t in the shadow of the shelf wires. Does that seem possible to you?
(If we’d arranged our spaces differently where I work, we might be having the same problem here, with afternoon sunlight bouncing off the floor and into tank-storage shelves)