An example being a deposit address on Faucetpay.io is not in the list
That's because the address was never used on-chain. I don't use Faucetpay, but I guess they use the Bitcoin address as "account number". Anyone can do anything they want off-chain, there's no way to index that.
Yeah I had a quick look at one of the addresses and blockchair has some history data, so I think they use it like a clearing account.
What I mean by that is that it is a real address available onchain but also offchain (some of my terminology might be a bit wrong but feel free to correct me if there's better ways to elaborate on how it works).
Offchain usage (or is that considered sidechain) is like the network of systems that can talk direct to the (in this case the one i looked at, faucetpay) network/system, allowing transfers to be fee free if the sending system chooses to do so (transactions don't show in blockchair in this case).
Onchain usage is normal real transfers via the block chain, registers in the historical data (eg on blockchair) and then at some stage (algo? time? amount? don't know) is moved out of that address to another address (hence the clearing) - but in the interface that I see the amount is still allocated to me. So my balance to my (faucetpay) address is buried in another address, hopefully readily available to me when i need to spend it!
Cheers