The article says that the workers were tipped off by these items once they traced back the mercury to her apartment (I assume the apartment building).
As to how these could tip off the workers, I think they used what is known as "The Exception Principle" which is an analytical or deductive tool used by investigators to find clues to an incident. The idea is to zero in first on something that deviates from the norm (or expectations), and obviously the tea-lights and Buddha shrine are unusual and exceptional in Austria.
But those items, per the article were inside the apartment