SRK wrote:
Hello,
I have not tested the 3-prong outlets with a circuit tester. The
building is from the 30's but the electrical system is much, much newer
(I can't be precise).

Not being able to see it from here , I think your best bet would be
to get an electrician to look at it.

If i was wired w/ x/3 and there aren't ground wires in the box, then
there shouldn't be 3-prong outlets. I suppose there could GFI
somewhere but can't see that, either.