But I don’t understand how the presence of tealights and a Buddha shrine could tip off the police to someone using mercury
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.
The article never really explained that. I thought they were using a little more scientific method ie tracing the mercury through the sewer pipes.
What really amazed me is the quantity of "healing" cream and powder she brought back. How could any person need that much product for any treatment and how is it that she could bring 10 kilos of these mystery substances through customs?