You invite a modeller friend over, and you see him taking a bottle of paint.....what would you do?

..."Hey....I bought that, because I need it".

Zero hesitation.

What goldhammer said. I'd confront him outright.

Two things:

1. Ask him to wear a helmet, because bottle number two will be pitched at his head.

2. Find a better friend.

Personally, Id just say  "If you need it, why don't you just ask."  Oh yeah, and that would be the last time he was in my home. If he will pack off a $2 to $4 bottle of paint, what else will his sticky fingers glom onto.....the better halfs jewelry maybe?