In this case, it's context based. The sentence can either mean "it/that" is a neuter pronoun or could mean gender is not implied. (i.e. "that" could be something that is happening - like an accident or a flashy car etc.) A lot of the time, "det" is used regardless of gender. Just remember, "det/den" are pronouns - they replaced the NOUN and therefore share the same gender as the noun that is replaced. I've spoken to a fair few Norwegians and they don't seem to stress using "den" too much. It's more grammatically correct, but you'll be understood either way.
Thanks a lot ! It helps indeed ! I'm afraid to make a lot of mistakes while trying to speak Norwegian, it looks to me as a sound interpretative language.
Apparently it is related to the morphology of the Norwegian mouth... is there a typical anthropometric factor among the Norwegians ? I came to this while doing Google Search https://doi.org/10.1016/j.wocn.2008.01.001