"Being as how" (with the meaning "since it is the case that", or simply "since") is usually found in informal contexts, especially in spoken English.
Not all speakers use it; you could call it non-standard. (It might seem old-fashioned, in British English; possibly less so in other dialects.)
In your example, it's slightly incongruous, since "notify the police" is a fairly formal phrase.
Either position is possible: it presents the fact that justifies a preceding or following statement.
Student or Learner