Is there a special marking for space marine scout squads?

[hd3]

Or is it just the number on the non chapter badge shoulder pad? I can't find any record or guide.

[Moonstalker]

From the official models I've seen (the old models were painted in Ultramarines colors, right pad has a white circle with roman numeral squad number inside, new models are Raven Guard with gothic script number on right pad) it's usually a number, presumably the squad number.  I assume because scouts would only be part of the 10th Company in a Codex-compliant Chapter, and thus did not fall into any of the standard designations of a Battle Company.

 

In older codexes, however, Scouts were considered a Troops choice (the equivalent of Battleline today, using the upward arrow icon) alongside Tactical marines.  In 30k, Scouts are still a Troops choice.  So it could be appropriate to give them the Troops/Battleline icon as well, especially if your chapter is divergent and integrates scouts into its Battle Companies instead of restricting them to 10th Company (like Space Wolves - or like my custom Chapter, which is more heavily influenced by 30k).

[Grotsmasha]

Generally speaking, it is just the squad number for the majority of Chapters.

[jaxom]

From Insignium Astartes

 

Scouts being Troops is a game mechanic. However, they’re your little plastic dudes; follow your inner rule of cool.

[Sky Potato]

The latest decal sheets have plain white circles for Scout Squads.

 

Presumably these are for introducing a white circle to give an area of contrast to place a squad number decal over - or you could just use a circle as the scout symbol in the same way as the arrow is for tactical squads.

[SvenIronhand]

Pretty sure at some early point, it was a combat knife symbol.

[Viridia]

According to Insignium Astartes, it's just a squad number (all examples in the book show the same pattern of Roman numerals inside a hollow white circle)

 

Edited March 23 by Viridia
Changing orientation of attached image