Not rolling is treated as passed loot. If the timer expires and no one has rolled (including passing) then it's gone.

If you're the only one in the instance (because everyone else left or you're soloing) then all loot is automatically given to you, unless it has a weekly restriction. In other words, you won't pick up current raid loot if everyone bails before you had a chance to roll, thus locking into an item you might not of wanted that week.