I've heard men addressing each other as strangers in public as "companero". I think, at least in this instance, it's an expression of respect. For instance, I heard a man address another as companero when he was asking for directions. Naturally, the whole exchange was in Spanish. :0)
It's similar to using 'mate' to address strangers in UK. They are not your mate for real, but it's just part of the culture to use such a word when referring to strangers. Here in Brazil I sometimes call strangers of 'amigo' when asking for directions (or any ordinary conversation), even though they're not really my friends.
Interestingly, on the discussion parallel to this one for Spanish speakers learning English, I noticed they frequently began their posts by addressing the group as compañeros. In this sense we are compañeros in the sense of peers as we are pursuing a common goal using the same method, ie, DL.
"We are not peers." would have a totally different meaning than, "We are not companions." Being someone's peer generally means that they are in your age group or in your class.
I've personally said something along the lines of, "She is not my peer." when speaking about a woman in her forties who I worked with. I'm only in my twenties.
Exactly. In American English:
Mates = Sexual relationship
Companions = Intimate relationship OR people participating in an activity together or traveling together
Buddies/pals = Casual friends of varying degrees of closeness
Co-workers/Colleagues = People in same professional group
Peers = People in the same professional industry or people of a similar age group usually when speaking of the school system
Acquaintance = Someone between a stranger and a friend, someone you really don't know but you recognize or someone who recently introduced themselves to you.
Someone who is your equal in rank, age, or social standing. i.e. In school, your fellow classmates are your peers, but you teachers are not. Your brothers and sisters are your peers, your parents are not. They are each others peers. If you are in the military, people of your same rank are your peers, people of higher or lower rank are not. If you are in your teens, other teens are your peers, but people in their thirties are not, and vice versa.