Sometimes I stumble upon a question where some close votes already have been cast (but the question is not closed yet). How I can vote for "Leave Open" now ? I can not search for a specific question in the review queue, can I ? The "close-dialog" on the question page does not contain the "Leave Open" option, as far as I can see.
You can't do that from the Q&A page, just like you can't vote 'leave closed' when there are pending reopen votes.
You can search in the Close Votes review queue with a bit of a workaround; you can filter by type of close vote (not suitable for this site, needs details or clarity, etc.) or by tag. Choose the least used tag (that's the rightmost one) and you'll have a good chance to find it immediately.