Sending city staff door to door to see if anyone is home is probably expensive, disruptive, invasive, and inaccurate if residents keep irregular hours. Sending snail-mail surveys is probably also expensive and would get a lot of false negatives. Can utilities services help? Does electricity use imply occupation? Does this give false negatives for households with low electricity use and false positives for empty houses with a few token appliances plugged in? What about natural gas and/or water usage? What about more indirect measurements like traffic/transit data or tax status (for rental housing at least)? Could telephone-pole mounted cameras measure foot and motor traffic?
(Disclaimer: this question isn't about whether unoccupied homes are a problem for a city, it's just a thought experiment about how this question could be answered with technology. I admit this question is inspired by speculation that empty homes in places like Vancouver, BC and SF are affecting rents and availability.)