It's very sad that you had valuables stolen from this restaurant. I can imagine they see an invitation through the window then take it whilst you are at the buffet. It's always wise to take your purse and mobile phone etc with you when you go to the buffet or make sure one person stays at the table then the next one takes their turn. From a similar experience (albeit a very drunken one in Barcelona one New Year) we never ever leave anything at the table anywhere especially a buffet place like this one. It's such a lovely restaurant with lovely staff who are in no position for a confrontation with thieves but I suppose you have to think about who is walking past unfortunately.