Like I say, I've been doing this for a year, and I've never come across a cache in a bucket.
Most caches are Ammo Boxes (that sounds bad, I know, but bear with me) - not only are these boxes usually Camo green, but they are usually also siuated in public areas, yet where the public doesn't normally go... and well hidden.
You don't *have* to leave something, but if you do it should always be inside the cache.