By seeing one address in a wallet, nobody can ever guess any other address.
Having multiple addresses is awesome, because you can isolate transactions from each other and by using a "mixer" you can send BTC from one address to another without anyone else even knowing you own both adddresses.
Generally, a wallet will send from multiple addresses in the wallet when it needs to (e.g. one address doesn't have enough to fund the transaction). That transaction will tie the addresses together.
Or the person is using "CoinJoin" or "Shared Coin" on blockchain.info. In that case, those input addresses are unrelated to each other at all.