When big pours of concrete happened no one stops for a body falling in. I think there are around six people in the Boulder Dam
Very freaky way to make an exit. The good news is it's probably very quick, like a minute or less.
I would suppose the goes for someone (careless or drunk) who falls into a vat of molten steel.
I worked in a steel mill for a while. It never happened at my mill. But it has happened.

I was a dam-builder,
Across the rivers deep and wide;
Where steel and water did collide.
A place called "Boulder" on the wide Colorado,
I slipped and fell into the wet concrete below.
They buried in that gray tomb that knows no sound,
But I am still around ...
The Highwayman - Nelson, Kristofferson, Jennings, and Cash
False. Concrete is poured in ‘flats’. Shorter cure time.
Nobody is buried in Concrete in any dam in the U.S.
The Bureau of Land Reclaimation claims that there were 96 fatalities during the building of HOOVER Dam but no entombments. Boulder Dam is what FDR’s Secretary of the Interior Harold Ickes (his excretable son by the same name worked for President Clinton) called it, but Congress never changed its name.
A body falling into concrete would create a void, and compromise the structural integrity.
https://www.usbr.gov/lc/hooverdam/history/essays/fatal.html
BUT-—AT LEAST THEY KNOW WHO THEY WERE>>.....
That is not really true. Having a body in a concrete pour will significantly degrade the stability of the concrete. It creates a cavity that will be a weak point. The engineers would never allow a body to remain in wet concrete.
Plus, workers are people. In the event someone dies on the job, they would recover the body in order to pay off the death benefits. Finally, it’s not “combat”. There is no reason to not stop a pour in order to save someone.
My guess is someone tossed a skull model into the concrete as a joke.