It's been connected to the internet, if I turn the internet off, is there a way my BTC can be stolen without physically taking my pc?
depends. where do you store your bitcoins? if it's on blockchain.info for example your coins are not safe, since it's an online wallet.
if you use electrum, bitcoin core or multibit - your coins are always safe. as safe as your local machine & passwords
Bitcoin armory, and electrum