Question: I have been involved in two volunteering groups for the past two years and I'm not sure If I should ever mention it in the SOP/CV.

Should I mention it in one or both my SOP and CV? Also, how detailed should I write on the type of activity/nature of the work/vision/reasoning/accomplishments, etc.

Note: I'm applying for Electrical and Computer Engineering for a master's degree.

The volunteering work is totally unrelated to the degree. The activities are focused on aiding homeless and abused animals

Answer: 
I would recommend against mentioning unrelated volunteer work in one's statement of purpose---after all, you have said that it is unrelated to the career that you are intending to pursue. If it *were* directly linked, then of course it would be useful to mention it.

Likewise, there is no space for this type of work in any of the main sections of an academic CV: this isn't like applying to college, where you are encouraged to list a bunch of extracurricular activities to show your breadth and interesting personality. Graduate admissions is instead more typically about showing that you have the mix of skills, drive, and focus necessary to succeed in you intended program.

It is often the case, however, that people will have some sort of "other significant experiences" section at the very end of their CV in which they put miscellaneous things that they think are important. If you feel that your volunteer work is an important ingredient in understanding you as a potential professional, then this is the appropriate place to include a brief note. It may help you if it catches the eye of somebody who finds it significant, and it is unlikely to hurt you.
I see nothing wrong in mentioning that on your resume even if it is unrelated. If you have enough place on your 1-2 page resume, you can put it.
In my eyes, It shows that you are a passionate, social and informed person and not just a (sitting in front of the computer) geek.