can someone explain how and where the companions (followers) spend the caps you give to them ?

I gave 500 caps to my companion because I am a good fellow and wanted to share wealth and reward him for his help.
Later in game, checking his inventory I found 484 caps, no longer 500. I know he grabs items on tables when he is not following and eats the food I give to him, but I never saw him buying anything.

Where did he spend the caps ?

the One Armed Bandit in one of the casinos I think