To opponents without a detector, this is untargetable, unattackable, and can sneak past patrollers. While patrolling, this is attackable.

"Sneaking past" means you can ignore patrollers when attacking and attack anything you want. You can also attack the patrollers if you prefer. — Sirlin
When something invisible attacks an enemy unit or hero, that enemy unit or hero DOES deal its combat damage back as usual. — Sirlin
If an opponent has the Tower add-on and their tower has already used its once-per-turn detect on something, then you attack with something invisible, the Tower will not deal its damage to your invisible thing. It would if it's once-per-turn detect hadn't been used, but it can only damage things it can see. — Sirlin
You can [target] your own invisible things whether you have a detector or not. — Sirlin