I've seen this happen once before - I think it's a bug (not sure if it's easy to replicate since the manner in which the situation occured had to be perfect timing and part the cause -at least this is what I THINK may have caused it- was a result of an enemy player's action). I've only seen it occur once, and the stealthy that carried it said that he wasn't stealthed on his screen when he ran it in - i.e. he didn't even know he had done it.
There are a few ways to do this, almost all of them boiling down to stealthing while in another animation and without breaking combat (the last part is key). The easiest way (and the only way I'm sure works) is to have someone use smuggle while a projectile you fired is on it's way to a target but hasn't hit yet. That works about 4/10 times and can give you a good 3 to 5 seconds of invisible ball carrying, more then enough time to pass of sprint away. Not sure on why it works, but it likely has something to do with how the client checking to see if the ball is dropped or not isn't the same as the check to see if stealth breaks.
There are posts on other websites about being able to do this in a repeatable way with the sent's invincibility as well, as it also doesn't drop you from combat, but I can't confirm that as I don't have a sent.
To be honest though, I bet 9/10 times people think this is happening it's just lag.
Valor 73, 61, 60, and 57 on 4 toons, not even counting my low levels who are all in the 40s, and I've never seen a hack or any unexpected player behavior I couldn't quickly identify as a known exploit with a quick Google search.