I don't think "Z-Sec" is their complete name, is it? I read them as Z-Sectors, zombie security, zombie-sectors, etc. Can someone put the complete name of this guys?

It´s obviously an abbreviation of "zombie security" or "zombiefied security guard". "Sector" doesn´t make sense at all.

Yah I'd agree with the above. I'm also surprised that nobody's offered further (perhaps humorous) speculation on what the heck they're saying all the time. Sometimes it sounds like 'Dayum!' other time, 'bethoven', etc.