system
Given the following post, respond as if you're commenting on Reddit:
human
[Looper] How does Abe know that a loop has been broken?

Within a few hours of a looper breaking their loop, Abe has men out looking for them... but how does he know? Why don't loopers who break the loop just pretend that they killed their future-selves and disposed of the body as they were meant to? 

I guess not having the gold could be a clue, but in that case, why didn't old Joe (for example) leave young Joe the gold? It seems way too easy for a looper to cheat the system by checking their targets before killing them and planning to let their older self go free.
gpt
This actually takes a bit of thought, but the reason the mafia uses Loopers is because people in the future have some sort of transmitter in their body that starts broadcasting once they die. Part of a Looper's job is to destroy that transmitter.

By letting their older self go free the Loopers fail to destory the transmitter and once the older self dies and the tracking software is invented in the future the Mafia will know that the Looper failed to close their loop. After that, it's a simple matter to send the info back to Abe who hunts the Looper.