Does anyone have any theories on how those zombies got to him? I mean, he was annoying and stubborn, but how did those zombies get him? I've been wondering for years how they got into the warehouse... Can zombies even open doors?

Yes, they can.

The door could only be opened from the inside and as far as I remember, it didn't look like the doors had been busted open by the zombies... Which means he probably opened the doors and decided to try to make a run for it. He didn't look for his daughter because he already mentioned he had lost her outside.

As for how the zombies got in... Maybe through the fence at the lower lvl of the area. You can't see inside properly but it looks like the fenced area might be accessable from the outside, somehow.

The woman you hear screaming shortly after exiting one of the first doors and going through a narrow alley, is Dario's daughter I believe.

I always thought that dead girl cuddling a dead UBCS soldier at the Clock Tower was Dario's daughter since there is a journal that supports this. It's the room with the three stone puzzle which needs to have all paintings read 12:00 I believe.

I knew Dario had a short life expectancy ever since I first played the game.

What section in said journal are you referring to, that would support her being his daughter?

That's what I always assumed. I never really thought about it more.

The file manhunter's talking about is here.
http://www.biohaze.com/files/re3.html In the Merc's pocketbook.

Damn, I never really thought about it like that. I just assumed there's a crapton of girls with fathers who feel very attached to the city, out there. :p

Thats sad :c i never read that file and didnt pay attention to the the dead people on the grounfd unless they were crawling towards me

He most likely got hungry after being stuck in there a few hours and went out to find something to eat. The zombies found something to eat instead.