They do; it's called Smartwater, but as a poster in another thread pointed out, if you chuck the cables in a fire (which apparently they do to remove the value-less insulation), you can say bye-bye to that.
Routine surveillance on the network is going to be a non-starter; how many miles of line would need to be covered? Intelligence lead surveillance and working to deplete the market for stolen copper is the only way this one will be won.