They're going to steal my coins that are on a machine running an open source OS, open source wallet, that doesn't have wifi?
OK, how...
Well, technically it is possible to see what is on the screen from a distance. See
Van Eck phreaking.
Besides, only absolutely safe way to store your private keys is a paper wallet, 'scrambled' with a
XOR chiper using a long sentence that you never reveal to anybody.