I own a 2003 sentra 2007 GXE. Last week The

THE coolant header tank has a tell tale light to let you know that you getting low in coolant and will switch off to protect your engine BUT there is a problem as the coolant system in good condition shouldent loose any coolant at all hot or cold...SO get the system pressure tested and fix the fault and also to be safe in months to come fit a new thermostat..
Let me know how you get on with all the above Ron

Sounds like it could be overheating. Especially if your wife is driving slow speed through town then there is less air going thru the radiator and the electric fan should take care of that. The reserve does not need to be full but should not be completely empty. When the engine coolant heats up it is pushed into the reserve tank when hot and then gets sucked back into engine when it cools. Anyways the first thing I would check is the radiator electric fan. If the fan is not working then the engine will overheat if stopped or at non-highway speeds. Check the fuse is good and make sure the fan relay is working. There is a seperate fan that works for the a/c and if you notice it start running rough then turn on the a/c and that will kick on the other electric motor to help cool the engine(just hope it is not winter time). You could let the engine idle for a while and see if the fan comes on. The mechanic using the computer has the ability with that computer to actually tell the car's computer to turn on the fan that way he can verify it is working. Sometimes thermostats go bad and can get stuck closed and cause an overheating issue also. So you could also replace the thermostat. Let me know if you have any more questions.