Compañero is also used to describe work colleagues: http://www.wordreference.com/es/en/translation.asp?spen=compa%C3%B1ero
I think you're right, I feel apart from business company, usually the root of and derives words from it origin suggests some closeness or friendship. Although languages can be funny with words meanings. Languages are alive so move with time and influences and when they migrate across borders they may take only one specific meaning, not necessary the most obvious from its original provenance.
My own understanding never really associated companion with work, but that's just me. However I have never heard anyone in my 48 years in Australia speaking of a work colleague and using the word companion. Yet after consulting several dictionaries the word colleague was mentioned in each of them....Go and figure. So I wonder if the same thing goes for Spanish speaking countries. And growing up in France I can almost say the same although I think in medieval time "Compagnon" may have been a kind of rank in one's professional formation. Yes, checking, resources, it was a stage between apprenticeship and fully pledge professional status. But as far as it concerned me it had a meaning leaning much to friendship... So this would have been my most obvious choice here. And in a lighter manner for this subject I would add that in Australia a companion with a good balance would have beer coming out of both his ears at the same time. Yes he would be on the level.
ha ha, I love this one! I wonder if it's related to something like "You know when Australians are on the level when beer is oozing out of their mouth equally on both sides. Or in France "well balanced women ' do have very equal 'pectorals' Ambos 'compañeros' tienen un buen equilibrio. But of course I could have misinterpreted the whole thing altogether, hmmm? Some don't think before they say something, other think too much....