It's old format address called P2MS[1]. If you see unknown, that means your wallet or block explorer missing few feature since you should see address such as "m-67202395617dc6a0a7e4eccba624f331".
P2MS doesn't even have an address format, so how could any explorer or wallet label it as such?
It’s the format of a multisig Bitcoin wallet address. You can check the definition below of a multisig address and it’s parts. It’s not unknown sender but rather a part of the wallet.
It's a very limited form of multisig that only allows you to use up to 3 cosigners because the data is not hashed before it is broadcasted, unlike P2SH.
Anyway, I find it very strange that P2MS's BIP was made just 2 months before the P2SH BIP. That's a very short interval where it could've been though out whether the former type was needed.