Two-thirds through an otherwise enjoyable cosy mystery, the main character discovers a photograph showing an object linked to a murder, suggesting that it was once in a certain house. Instead of taking the photograph to the police, even though the police are in this case sympathetic to her cause, she goes to the house to check if the object is still there.
In part cosies require characters who will poke and pry and get involved in other people's murders. But it's a strong negative for me to follow a character into unnecessary danger in the face of a more obvious step.
The rest of the story worked, and I'll read further into the series. But TSTL syndrome is something I definitely hope to avoid in my own writing.