New Yokosuka Harbor
When you get off the bus, there's only one way to go - into the harbor. Just past the security guard station, you'll come across two thugs intimidating a third guy. Ryo sticks up for the mugging victim, and has to defend himself from the leader of this two-man gang, Goro.
After you deal with him, Ryo will ask Goro if he knows where Warehouse #8 is - Goro directs you to go down the road you're on, and take a right. You'll have to go past the side of another warehouse before you can make the correct turn, though there is a dead end with a truck parked in it just before the warehouse turn.
When you go to walk through the main entrance to the warehouse though, the guard will stop you - there must be another way in. Head around to the back of Warehouse #8, and Ryo will look up at an open window - but he'll need a boost to get onto the crates that will let him reach the window. Luckily, there's a small crate on the ground to the right of the window. Walk over to it and push it towards the larger stack of crates, the press A once you've got it all the way over. Ryo will climb up to the window, and you can get into the warehouse.
Once inside, go towards the offices on the right hand side. You'll overhear a couple of dock workers talking about a mail mix-up; they've gotten more mail that was meant to be sent to the Warehouse #8 in an Old Warehouse District.
This new #8 doesn't seem to be the place, so it's time to investigate the Old Warehouse District. Back outside, hop down off the crates and go out towards the main wharf. On the right hand side, Hisaka-san is running a small sandwich stand. Ask her about the Old Warehouse District, and she'll point you along the waterfront. Take a stroll along there (it's quite a distance), and you'll come to a guard standing outside the gate to the Old Warehouse District.
Trying to get in will result in more shouting from security, so Ryo resolves to try at night - which means more hanging about. It's worth learning your way around the harbor, though - there's more than one way to get to most places, and it'll be beneficial later if you're familiar with the harbor layout. You can also go to Warehouse #4 to practice your fighting moves, play darts in the Harbor lounge, or spend money in the Tomato mart.
The final option is something of a side-quest - you can go back to Hisaka-san and speak with her to find out about some trouble she's having with her little sister, Mai. She's been hanging out with some unsavoury types, and Hisaka-san will ask you to talk some sense into her.
Go down Amihama Wharf towards Warehouse #17, and you'll see an old homeless man being kicked out of a warehouse by two security guards. If you speak to him, he'll ask if you could buy him a can of coffee. There's a vending machine a little way back, so if you can spare the 100¥ you can get it for him.
Once you've done that, go down behind Warehouse #17 to find Mai and her friends - those sullen schoolgirls who hang out around Dobuita looking shady. After telling Mai to come with you, Ryo will have to beat up her friends.
A short guilt-trip later, Ryo leaves Mai to think about how she's treated her family, and heads back towards the main harbour area.
Go back to Hisaka-san and you'll get another cutscene. She needs to make a delivery to the Old Warehouse District before she leaves for the night. If you agree to make the delivery for her, Ryo will go across to the Old Warehouse District and discover that security in the area is lighter between 8 and 9pm - that's your best chance to sneak in.
Old Warehouse District
Once it hits 7pm, you can go back to the Old Warehouse District. The security isn't as heavy after 8, but you can get in now anyway - and it's not too difficult either way. The guard will no longer be standing outside the Old Warehouse District, and they've kindly left the gate open for you. It's not going to be that easy, though - you've still got to sneak past the guard station, for starters.
If you miss that QTE, you'll be automatically teleported to 7pm the next day - and will keep being pushed forward a day every time you miss. There's no rush to complete the game, but there is a nasty surprise if you end up in mid-April. If you want to see it, make sure you've got a recent save file first.
Once you're inside the Old Warehouse District it's not much easier - you have to make it past several patrolling guards to find Warehouse #8.
If you get caught you'll go to 7pm the next day and have to repeat the above QTE. After a few times messing it up, you'll get a blank map of the Old Warehouse District from an old homeless man who lives in the harbor, which Ryo will fill in when you look at any of the warehouses. Keep screwing up, and he'll eventually show you the guard patrol paths, then will tell you when their security is lightest.
To get to Old Warehouse #8, you need to wait until the nearest guard turns his back to Ryo and begins to walk away, then run down the path he just came from. Take the first left after that, just before a large stack of crates, the take the first right.
You'll now be outside Warehouse #8, so head in to find out just who this Master Chen is.
Old Warehouse #8
Inside, there's a lot of shelves - items stacked with antique pots and plates, and a lot of crates behind it. One of the plates stands out - it's much whiter than the others. Pick it up and the lights will come on. You'll be noticed by a man in a suit, Gui Zhang, who demands to know who you are.
He's stopped by a much older man in red robes - Master Chen. After you show him the letter, he offers to help you. He seems familiar with Lan Di's organization, the Chi You Men, and asks if only one mirror was stolen. It soon becomes apparent that your father brought two stone mirrors back from China - Lan Di only took the Dragon Mirror, but a second artifact may have been in Iwao's possession, the Phoenix Mirror.
Master Chen tells Ryo to find the Phoenix Mirror, advising you to phone ahead next time so you don't have to sneak past the guards. You'll then be taken back home, to save and sleep.