Section 73 of the British Columbia's Liquor Control and Licensing Act is the relevant law...
Unlawful possession or consumption of liquor
s.73(1) A person must not consume liquor, or possess liquor in an open
container, in a place other than
(a) a residence,
(b) a private place,
(c) a service area in respect of which a licence, authorization or
permit allows consumption,
(d) as provided under section 11,
(e) as provided under subsections (2) to (4) of this section,
(f) an assisted living residence, community care facility, hospital or
other prescribed facility as provided in section
(g) in a liquor store as allowed under the Liquor Distribution Act.
(2) Subject to subsection (3) and the regulations, a public place, or
part of it, may be designated, by a bylaw of the municipality or
regional district that has jurisdiction over the public place, as a
place where liquor may be consumed.
(3) A bylaw under subsection (2) must contain the hours during which
liquor may be consumed.
(4) Without limiting subsection (2), regulations under that subsection
may provide that a bylaw referred to in that subsection may not
(a) a specified public place, or
(b) a specified public place for a specified period of time.
If a single can of beer is missing from a six pack it is illegal?
No, as long as none of the cans are open.
If a bottle of hard liquor has been opened and resealed, it is illegal?
Not in its resealed state, but the act of opening the bottle may be unlawful depending on the circumstances.
What is meant by "in public" in this context
Everywhere not accounted for in s.73, above.
If I'm walking down the street with a two six of vodka in my backpack that I had already drank from, would that be illegal?
Not if the lid is on the vodka bottle as it's not an "open container". When and where you drank it is another matter.
Does it matter if it's concealed?