An expansion tank never hurts, but may not help. Where I live, it is a code requirement, even if it technically doesn't 'help'. Most utility companies are installing check valves on their supplies to buildings, and once that is done, an ET is required or the tank's safety valve will often open because expanding water has no place to go. This may or may not be a huge issue in a big complex, because all it takes is one leaking valve, and that expansion might be released without raising the pressure.
There are lots of water sensor alarms out there. There's a fair amount of valves that can close the incoming water to the tank after the sensor says there's a leak. The only one that I've found that does not require power is the WAGS valve. It DOES require a pan so that the water level can rise (3/4") to trigger it. Many of the water alarms will trip at a lower level. That assumes you have power to make them do something.
Often, the simplest solution is also the most reliable. The WAGS valve is pretty simple. It's a one-shot deal...once triggered, it must be replaced. A water alarm should be tested periodically...how often do you think that will happen in a condo situation? Alarm fails, somebody unplugs it, or forgets to replace the battery, and no protection. That assumes that the valve works, too, which also requires power.