+1......do not, I repeat Do Not store in a water/air tight cover with a drawstring around the bottom for more than a night or two. They Don't Breathe allowing moisture to escape. They will keep the bike from getting soaked in a heavy rain, but the moisture will get trapped inside and eat away at the bike over a short period of time. If you Have to (you really shouldn't) leave it on a balcony cover it with a tarp, but don't drape the tarp over it. I would probably strip it down as much as possible ie. tires, tubes, chain, etc.
Actually F*** what I said, grow some balls and take it inside, you'll be glad you did.