Manhunt by Kate Messner

Valuable paintings are being stolen from art museums around the world, and no one has a clue about the thieves. Henry, Anna, and Jose are junior members of the Silver Jaguar Society, and they've solved some mysteries in the past. They travel to Paris with their parents, full members of the society, where they meet up with Hem, the son of another member. They discover the Serpentine Princes are behind the thefts and proceed to conduct their own investigation when their parents disappear. The society thinks one of its members is giving secrets to the Serpentine Princes, so everyone is a suspect. The adventure to save the artwork takes the characters all around, and below, the city of Paris.

I believe this is the third book in the series about the Silver Jaguar Society, but they can be read out of order. There were few references to previous books to confuse readers. The missing artwork, secret messages, and sneaking around the city created a nice a mystery and adventure. The unknown mole in the society added a bit of suspense.