Paul Farquhar
Legend
Yes, I would rule that any use of the help action would revel the imp's approximate position to the target. The imp might try moving, but I would require them to make a stealth check and move at half speed in order for the target to not know their new location.Just pointing out throwing rocks would be an attack (IMO anyway...).
But, yes, you could have the Imp making a bunch of noise, kicking up dust, or whatever.
Also, the point about being within 5 feet was in response to throwing rocks, not something generally done within 5 feet.
All the Imp has to do is move within 5 feet of the target, use the "Help" action (however the DM or player narrates it), and then move away. It doesn't have to be visible to help. We are all agreeing on that I think.
And intelligent creatures might well have the same invisibility countermeasures players would use.