Is Praying to St. Anthony Idolatrous?
if we lose something and pray to St. Anthony, is it idolatry?
No, because we're asking St. Anthony—or any other saint for that matter—to intercede with Jesus on our behalf. Indeed, when we ask others for prayers on our behalf or for others, we can do no better than a canonized saint in heaven.
In summary, saints don’t compete with God, they collaborate with him, who is their creator as well, in advancing his kingdom. For more on praying to the saints, see this Catholic Answers tract and also this one.