Perhaps you're thinking of a similar idiom like:
* "The trail is going cold" This means that the investigation or search for something is becoming less promising, with fewer clues or leads.
* "The trail is getting cold" This has the same meaning as the previous idiom, but emphasizes the ongoing process of the clues becoming less useful.
Example:
"The police are worried that the trail is going cold in the missing person case. There haven't been any new leads in days."
If you could provide more context about where you encountered this phrase, it might be easier to understand what you're trying to convey.