Yes, at times it do happens like this and it is not only coinomi Wallet that do act like this, Imtoken and Trust also do like this once a while and like you said, one can always scan his address to check new tokens that has
being sent to you without you receiving notification.
There are actually times that this happens not only to the wallets you have said, but it also happens in the official wallet of ethereum.
Yes, and it can be tracked using etherscan and you will also find information about the coin that you were about to add in your wallet.
This is only natural things to happen in erc20 or created token using a smart contract.