Please do note that you are not protected by Rule 4 as a Thief, as you are not a traditional antagonist. You are not allowed to kill crewmembers, nor commit grand sabotage unprovoked.
Thieves will come in different flavors, and their objectives depend on the variant.
Total classic, steal a High-risk item and escape.
Hoarders have a problem, and refuse to change or get help. Gets objectives to hoard large amounts of valuable items like insulated gloves in a specific area in maintenance. Temporarily disabled due to being unfun for the thief (the best way to get the items was to order them from cargo).
Essentially the survivalist but without kill permissions. Steal a lot of guns and get out alive!
Steal organs and keep them in a healthy shape up until the end of the round. Harvesting from players is probably fine as long as you let your targets live.
Steal someone else's Family Heirloom! Your own won't count. Take a look on the medical records to find people who have heirlooms.
Like Hoarder, but with corpses. Remember, no killing!
Steal a ton of IDs. Only IDs people spawned with count for this.
When the first thief is created, there is a 20% chance of a thieves guild being chosen. This is an abandoned room in maintenance all thieves know of, for meeting up. Thieves get a big mood boost from hanging out there!
Petty thievery is not so harmful, so try asking a cyborg or bribing personnel to gain leverage towards your objectives.