Because horizontal slots are normally not made
That is not true.
and nobody suspects anything there
Everyone should suspect something here, because after all, the area in question is an installation zone according to DIN18015
so you can quickly drill into it.
If a craftsman drills into it, he has to pay for the damage, because drilling is not done in installation zones without prior inspection!
And if the client does not know where his cables are located, he should not drill in the installation zones either.