what does "the trail is cold "mean?
For a minute, we all just look at one another.
“Huh? You stole it?” Leo says, recovering first. “But why?”
“I collect such things,” Angelina replies, ducking around Leo to keep an eye on the Scouts.
“Do you still have it?” I ask.
She shakes her head. “Lost it years ago in a fire.”
Leo and I hang our heads in defeat.
“But it wouldn’t matter anyway,” she continues. “There was plenty in there about the feud, but nothing after. Unless Ellerby and Fitzpatrick kept journals themselves, the trail is cold. And the only things I’ve ever seen from those two are business ledgers. Why does this matter to you anyway? It’s ancient history.”

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Answer

The terms "hot on the trail" and "the trail is cold" refer to a chase, or one thing following another thing.

If the trail is "hot," that means you are close to "catching" or "finding" what you're looking for.
If the trail is "cold," that means you are not close, and there is little hope in finding it.

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I think this term comes from hunting culture. If a hunter found an animal's trail / path that was still warm, it meant that the animal was nearby. If the animal's path was cold, it meant the trail was old and the animal had gone too far ahead.

Answer

When a "trail goes cold", it means you can't find any more clues to solve a mystery, specifically from a particular lead.

Example:

"I was trying to figure out where she was travelling, but I couldn't find any plane tickets after she got to China. The trail went cold."

"I was asking around town about the suspect, but nobody seems to know anything about him. The trail has gone cold."

Answer

Its a metaphor . if an animal has just past, its trail is still warm, and signs like foot prints, or scent, or pieces of fur will still be there to indicate where the animal went. It will be possible to track the animal.
Trail means to allow to drag or stream behind, as along the ground . and cold with a low temperature or a temperature that is lower than normal . So this metaphor means the path that you follow , so may be its means the cold is old

Answer

When what you are following stops giving you information.  What you are following always gives information such as money transactions, footprints, a smell, there will always be a trail, things left behind... you follow these to find what you're looking for.  But sometimes the trail stops and it's very difficult to find it again, and you search for it and you don't find it; the trail has grown cold.  You lack the information you need to continue following.  This expression comes from tracking and trailing people in the old cowboy days.