After using tAda on the testnet, what happen with unused remnant if for any reason it is not returned to the testnet faucet ? what are the technical implications of not return it to the faucet?
There's no harm in holding small amounts of Ada in wallets for personal usage. If you're not going to play on the testnet anymore we ask you to return funds to the faucet. A few thousand isn't going to make much of a difference in comparison to 30 billion or so in the total system. If you're holding large sums though, please backup your wallet keys and return the funds if you retire a testnet pool, as a number of pools that have delegated stake don't make blocks anymore hurting the chain density on testnet. If we lose too much, we'll need to reset the public testnet from genesis again.
My guess is that the tADA will remain in test wallets and the faucet will eventually run out.
Projects that are testing and accumulating a lot of tADA (eg a DEX) should send it back once they have completed their testing.
Individual users should send back their tADA if they are not planning on using it any time soon, and request some again when they do need it in the future.
If it doesn't get sent back then eventually a new token would probably need to be created and used for testing.