Thank you, Frank and others. The issue is solved, the curious may read on.
I figured that it had something to do with the sign. The prefix - is logical if you think of it as a number in binary notation, but I thought of it as a bit field. We have a large amount of legacy code that compiles with the Prospero Extended Pascal compiler. I do not know why it does, maybe that compiler is not standard compliant in this regard.
That code consists of a library that stores flags in 32-bit constants. Then, in many utility programmes, some of these flags are ORed together and stored in an integer variable (although a cardinal would have been more appropriate). In our case, the flag that is stored in the 32nd bit (the sign bit of an integer) never occurs alone, and therefore the hypothetical value of -0 is never attempted to be stored in the integer variable. Actually we do not have 32 flags, so we can just reorganise the bits and not use the 32rd one.
Best regards, Bastiaan.