I have a Rigger . I've equipped his GMC step van with a sm drone landing rack. now I am wondering, what does it look like ? where can it be mounted? if I park a drone in it can every passer by see the drone?

1: depends on the type of drones you're using it for (flying, ground, anthropomorphic)

2: see 1, on the roof for flying, under or on the side for others.

3: standard racks I say yes. the rules leave this ambiguous, I'm sure you could purchase/install concealed racks.
drone racks on a stepvan are probably not terribly unusual on vehicles driving around the sixth world. I could see them being widely used for maintenance and cleaning. drones with big farking guns or other mean-looking modifications (ruthenium, for example) might raise a few eyebrows, and cause a few "you got a license for that?"s.

My SR3 rigger had a van and the loading rack was on the inside cargo space (steel lynx model drones). I could load 4 at any given time (I owned 5 or 6) and they each had a coffin hotel like space: two bunks on either side of a middle walking area.

so, in theory, for a nissan roto drone, i could do a "turtle" like container on the roof. or a drop down in head room in the back with a hatch on top?

Personally I'd say you could do either. If it's on the outside then I suggest making it look like one of those cargo thingies that exist today.

Oh here, the X-Cargo thing.