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Background
You'll trigger this task when you interview Garte (#1) about the hanged man, and ask him who called the police.
Walkthrough
Garte will mention that Sylvie, the regular bartender, hasn't been to work lately, and he'll speculate that she might be the one who made the call. But when you contact Sylvie (using the radio in Kim's car, #2), she'll deny it. She'll tell you that since the Dockworkers' Union knew about the crime, and since they're the de facto police in Martinaise, she didn't see any reason to report it further.
This will leave you stuck without a lead, and you won't get anywhere in the task for a long time. But eventually, after you've completed the task
Speak to the Assault Victim, you'll be able to talk to Klaasje on the roof of the
Whirling-in-Rags (#1). She'll tell you that she had amicable relations with the victim, and that she didn't like seeing him hanging from the tree behind the hostel. You (or Kim) will realize that Klaasje must be the one who made the call, and she'll confirm it, which will complete the task and earn you 10 xp.
1 - Whirling-in-Rags
2 - Kim's Police Cruiser