From what I have read this term refers specifically to armed thieves who steal from people who have just taken money out of the bank. The normal word for those who steal like this (in America as well as the UK, I believe) is "mugger". I don't know of a specific term for those who do it in banks, but such people are commonly referred to as muggers.

"Son 30 las bandas de delincuentes en Lima que asaltan como ‘marcas’
Puede ser una pareja de enamorados, una mujer con su hijo o un tipo elegante que aparenta realizar una transacción o pagar algún servicio en un banco. Las bandas de ‘marcas’ suelen mimetizarse de estas formas para acechar a los clientes que retiran sumas fuertes de las ventanillas.
La víctima, tras dejar el banco, es seguida sigilosamente en un taxi o en un auto particular. En el camino, aquellos sujetos se contactan por celular con sus cómplices para indicarles el recorrido para luego desaparecer de la escena.
Seguidamente entran en acción los delincuentes quienes, pistola en mano, emboscan a la víctima en cualquier calle para despojarla de su dinero."
http://elcomercio.pe/lima/900304/noticia-son-30-bandas-delin...

Muggers often work in gangs. In order to highlight this feature of "marcas" and distinguish them from individual opportunist armed thieves in the street, perhaps we could say "organised muggers". The prior surveillance and the use of disguise or assumed identities are fairly common practice among mugger gangs.


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Note added at 3 hrs (2011-12-22 21:45:31 GMT)
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The term "spotter" certainly corresponds to part of the modus operandi of "marcas". A spotter is either a lookout, whose job is to watch for the police and warn the other criminals if they approach, or someone who identifies potential victims, but the spotter himself (or herself) does not actually commit the robbery.

"spotter
A criminal-accomplice whose job is to watch out for incoming policemen while his friends commit a crime (usually drug dealing, burglaries, safeguarding supply houses, etc). Spotters are basically human alarm systems that criminals use to warn them when the cops come near. They usually stand on a corner to get a better vantage point and to give their friends plenty of time to run away, flush the drugs, etc. The warning is usually a shouted code, but can sometimes also be visual; modern criminals now use cell phones. Often times, spotters are juveniles who can't be effectively punished by law for aiding in a felony."
http://www.urbandictionary.com/define.php?term=spotter

"Victims said the robbers are a white and a black man, and that they are working with a third man, a "spotter" who helps them zero in on victims.
Once the spotter sees a likely victim, usually someone walking alone, the robbers swoop in."
http://www.silive.com/northshore/index.ssf/2011/09/armed_mug...