Your experience in enforcing the law, and dealing with lawbreakers, gives you a feel for local laws and criminals. You can easily find the local outpost of the watch or a similar organization, and just as easily pick out the dens of criminal activity in a community, although you're more likely to be welcome in the former locations rather than the latter.
Use the tables for the soldier background in the Player's Handbook as the basis for your traits and motivations, modifying the entries when appropriate to suit your identity as a member of the city watch.
Your bond is likely associated with your fellow watch members or the watch organization itself and almost certainly concerns your community. Your ideal probably involves the fostering of peace and safety. An investigator is likely to have an ideal connected to achieving justice by successfully solving crimes.