Tobin was secretly a former cop from Albuquerque, he keeps his identity a secret because at the start of the outbreak, people looked to him to lead as he was an authority figure, and people died under his leadership and he didnt want people to look up to him in that way.
One day ( after his past is revealed ) he could tell Carol a story about he once arrested a kid who was throwing thousands of dollas out of his car ( Jesse Pinkman ) This would link breaking bad to walking dead forever and be so cool!