"The thief hides gems under his toga."
Translation:Fur gemmas sub toga celat.
Ablative case; from one source, Sub takes Ablative or Accusative — Under (accusative if in motion, ablative if still) http://latindictionary.wikidot.com/portable:latin-prepositions
I am a Latin rookie, but in many Romance languages no posssesive pronoun is necessary when referring to oneself. Unless a possessive pronoun is used, It's assumed that the reference is to the subject. I think Perce Neige's comment is correct - if the word "his" is inserted then it would imply the subject is hiding gems under a different man's toga.
It can also mean that, and also the black robes which judges tend to wear; but in this course, they generally mean long (about 4–5 metres long by 1.5–2 metres wide at the widest), semicircular lengths of wool fabric which were wrapped around men in a specific way and which constituted the Roman national garment — approximating what a three-piece suit would represent in today's Western civilisation. Look for any statue of a Roman emperor or senator and you will see them wearing it.