Doing a search for "what is the buoyancy of a human body in concrete" is probably not something you want in your work browser history.😄

Standard concrete is more dense than a human body, so you won't completely sink. There's a type of concrete that has been used experimentally at airports and runaway truck ramps that has tiny styrofoam balls mixed in it, so it goes "smoosh" when something hits it. You might sink in that.

As mentioned, concrete is alkaline and exothermic when curing. But so are plaster casts, and people used to get them all the time before fiberglass started taking over. Some stockinette and cotton rolls should work, though you may need a layer of plastic wrap to keep the moisture from seeping in.

You'd need some way to breathe. A mask with tubes to the outside is probably safer than a mask and oxygen tank, since the tank will run out at some point. You also need to consider your other bodily functions.

Unless you injure yourself when falling in, you shouldn't bleed. Concrete contains rough rocks called aggregate, so you can definitely cut yourself if you move about violently.

As to being found, it depends on how people are looking. If you're in a concrete block being cast for use as a movable barrier, the crane operator may notice that the block you're in is lighter than usual.

Someone with ground-penetrating radar will spot you as a low-density void in the concrete if they happen to pass directly over you with their unit.

If you can't move, you can't escape. I doubt you could swim your way out of concrete. As it hardens, assuming you've taken precautions about the alkalinity and heat, you'd be completely stuck and immobile. You might have trouble breathing as every time you exhale, concrete will try to fill the void left until it cures, making it difficult to inhale.

I've done something quite similar, so the above answers should be spot on. This is in the family-friendly section, so if you need more details, let's take it to private messaging