Yes, that was a phantom hit, and here's exacly what it is.
Each character is composed of polygons and boxes, as I'm sure you know. Boxes (i'll call them character boxes) tell the game where your character "is", while polygons and texture are the cool details we see while playing.
When you use an attack, it puts out what's called a hitbox. If your opponent's character box is inside your attack's hitbox, the attack connects.
However, if the character box and hitbox are tangent (IE only touching along a single line. No overlap), a phantom hit occurs. Any "animation" the attack has will still play (for example, if a Falcon phantom hits with a knee, you'll still see the electric animation on your opponent. You of course already know this, because you saw it with Luigi's fire punch). You'll only do half damage, and there will be no knockback or stun on the hit.
To see a phantom hit, try finding the video of Luke (bowser) vs Eddie (Shiek) on youtube. At one point, Luke yells out "What the f*** is going on with these phantom hits!?!". Rewatch the part right before that (a fair from bowser), and you'll see the phantom hit clearly. You can hear Eddie claim the phantom hit was indeed not a phantom hit, but his hax DI afterwords, which is quite funny
EDIT: I'm not so well informed on ghost hits, but if you're curious, I think I know how to reproduce one pretty easily. Do Ness' yo-yo glitch, equip any jacket, then hit your opponent with it. Now, attempt to use your upsmash on your opponent. It'll go right through him, as if you hadn't done anything.
I don't know about "stun with no knockback" though. Haven't heard of that. I've always heard that aformentioned side effect of the yoyo glitch to be a ghost hit
) and when I came around for another go, I did a dash attack and I went straight through Fox.