Hi Eleena. I can't say I've seen anything like that before, and I've come across all sorts of things, as you may imagine.
I would scratch it to see what it's made of.
Old lead-covered wiring
Lead water pipes
If it's where you might find a wall light, then it could be remnants of gas lighting. That would be lead as well.
A gentle scratch would tell you if it was steel, in which case you could tell by how hard it feels.
If it's lead, you'll soon know by how soft it is.
I did think it might be steel reinforcement. It does look like concrete from the picture, but it's rather too near the surface for that.
I'd love to be there to see for myself, but anyway, no problem in simply covering it up. Perhaps we'll never know.