I've not touched a large part of my online wallet (two weeks), nor the paper wallets under my sink (two years), but they are by no means disappeared, idle or lost. There's really no way of telling, no matter how much thought is put into it.
Some coins can be called lost, that is they don't have any associated private keys known, but there's no way of verifying that they are indeed unspendable. The exceptions are spends to addresses that the private key was never known like 1BitcoinEaterAddressDontSendf59kuE (1.6BTC).
There's an almost constant stream of suggestions that "idle" coins should be recycled, but really, what constitutes idle?