On Pic 3 and 9 of the Nintendo Switch gallery the bottom left seems to be "missing" something or it is white/grey plastic.

Maybe it where the contacts for the docking station are hidden. If its a moveable piece of plastic maybe it could be used as an additional air intake while being within the dock.

But for that it would make more sense if it would be on the other side as the outtake vents...).

On the other side cooling the batteries too while loading can't hurt at all...

I would assume active because the gamepad thing that Nvidia sells has a fan in it.

There seem at least to be TWO different version of the dock (Pic1 and 9 of the gallery) of which one (pic9) has less depth and lacking some connectors (USB?) on the side.

That dock "lite" makes it more likely that there is only one fan in the whole system... . But we still have to see the backsides of all docks.. . 

Or maybe there is only one dock and the angle makes it look different.

It is active cooling, no doubt about it. The switch packs some serious punch for a mobile device, that just isn't possible with passive cooling. There will be a passively cooled revision some years in the future though, so if you really really can't live with a fan, just wait some time and everything's fine.

If this is 14nm, then it will be a looooooooooong time before the next node is ready for prime time. IOW : this is probably it for the life of the model.

There could be two rectangular and one circular holes on the bottom of the front too...

It might be actively cooled... though it's really a question if it needs to be in handheld mode.

Pixel C has 20nm X1 inside of it, with max clock of 850MHz (that's 435GFLOPs for those who care) and it's just a regular 7mm thick tablet.

If NS has 14nm X2 (or whatever it's called) SoC inside, given that it's 14-15mm thick and has those air vents, it's possible it might not even need active cooling while handheld.

Indeed, the 3DS can get pretty warm too with certain games, but as long as the durability isn't affected with the fans, then it should be okay