I don't think it's a proper name, despite the capitals. I can't find any evidence that it is, and people throw capitals around in texts to make things sound important; you can't read anything more into it. So I think it needs to be translated.

There's no doubt at all that despite the lack of dictionary corroboration, "destacamento" in Central America can a facility where a "destacamento" of troops is stationed. Many texts that make this clear could be cited. Here's a map showing one, in San Juan Cotzal:
http://wikimapia.org/#lang=es&lat=15.435828&lon=-91.028987&z...

This example also shows that they can be quite small, and I think this was probably the case with the one in Comalapa, though I can't find any clear evidence of how big it was. But it does seem that it was not just a place where soldiers lived but a facility designed for controlling the town and surrounding area; in other words, not exactly a barracks:

"En la entrada al pueblo, el ejército colocó un destacamento militar, que controlaba la entrada a la localidad, y en donde ingresaban los detenidos de Comalapa y de otras regiones del país."
http://cmiguate.org/kaqchiqueles-desaparecidos-en-destacamen...

"En donde entraban", referring to the "destacamento" makes it clear enough that it's a place.

So although most reports on this in English call it the "former military garrison" or the "former military base", I would personally call it a "military post". But there would be nothing wrong with "garrison", I think. "Base", to me, suggests something quite large and is probably less suitable.