Translation:They understood that something bad had probably happened.
The partitive means something bad of the all bad things. Some particular thing from the possible bad things. https://en.wikipedia.org/wiki/Partitive
Similarly: něco vody - some water, něco chleba - some bread, něco peněz - some money
Related to that is the negative genitive that (unlike Polish or Russian) only works in certain partitive use cases. Especially after nic (which you can regard as an adverb of quantity). Otherwise it is archaic (Nemám peněz.). It remains in some fixed expressions (Není slyšet vlastního slova. Oka jsem nezamhouřil.)
I totally agree with LetitiateT. I am not reporting incorrect/missing version, I'm just asking why the word "wrong" could not be used in that sentence.
Generally, I do not want to be impolite, rude, offensive, whatever .. but I would like to know where I have done a mistake or why that word could not be used. I don't want to use the Report button because I'm not sure if the sentence is correct or not.
Let say I use the Report button. In case that my suggestion is incorrect (and honestly, I'm quite sure that most of my suggestions are incorrect), will I get some explanation why my suggestion is not accepted? I do not think so. So, finally, I use report button and still do not know if I'm right or not (or where I've done the mistake).
I hope I described clearly my point of view.