"Some police officers are coming."
Translation:Des policiers arrivent.
You are right, "certains" is often used to translate "some + plural subject noun", especially when the context distinguishes a few individuals from a group, like:
"Some police officers are coming but a few are on duty".
The original sentence was "Des policiers arrivent", which usually better translates to "Police officers are coming", because the only meaning of "des" is "more than one".
Most of the time, "des" does not need a translation and using "some" does not make the noun or the sentence more accurate, but often more confusing, in my opinion.