If a thief successfully steals something from you, he/she is still protected by guards, because your
character or guards didn't notice this. You can attack the thief if he/she fails at stealing.
You can notice an failed steal attempt by these things:
- A guard says something similar to this: "chaosgoblyn, seems like Opera is interested in your stuff". I guess in this case the thief got your item(s), but he/she becomes unprotected.
- A message appears in your "infobox": "Someone tried to steal from you".
If you won't see these and thief got your item(s). It is just bad luck for you. But think positive, the thief became happy.

Thief becomes protected again after he/she dies (or stays alive for 24h?).