Quick answer
Official package-thief objective
The official Roblox description says package thieves are part of Mail or Jail's delivery pressure. Watch the delivery flow and protect the run, but do not rely on undocumented tells, spawn logic, or penalty values.
For mail or jail package thieves, the documented objective is simply that package thieves can interfere with deliveries. The official source does not publish thief tells, catch mechanics, locations, or penalties, so this guide keeps those details unknown instead of guessing.