What actually stops The Whistler
The badge text is the only first-party rule Dread Forge has published for this threat.
“Stay inside while it whistles. Run back into the house if it spots you.”
Read that text as the whole allowed answer: Exterior of the house is where The Whistler acts, and the rule above names the action that answers it. Nothing in the official text gives a number of seconds, a spawn chance or an audio cue, so this page does not invent one.
1,801 players had earned THE WHISTLER at the last reading, which tells you the encounter is common enough to plan for and rare enough that most players have not solved it.