Coolant level is below "low" in the expansion tank, dealership service says this is normal?

There is nothing inherently wrong with what the service manager told you. (As an aside, service managers and mechanics are looking for add-on work. They get paid more money for this work than work you bring to them to do. If he says it's not a problem, believe them, because if they could make some money off of you, they'll do it.)

If it serves your peace of mind to add some coolant to the overflow, there is no issue with this. Follow the vehicle owners manual as to when to check it (hot or cold) and add fluid accordingly. Anywhere between the marks is golden. Check it every so often to ensure you have the right amount of fluid in it. If there is an issue, it will all of a sudden be too high (over flowing high) or too low (bone dry). If it does this, then worry about something being wrong.

You can add distilled water (which you should use instead of tap water ... tap water will have minerals in it which can react with metals in your engine and radiator and cause sediment in your coolant system). I think a better choice is to purchase 50/50 pre-mix antifreeze/distilled water. Purchasing a gallon of it, topping off your system, then leaving the rest for a later date when you might need to add a little more. My understanding is there is no shelf life on antifreeze, so put the top back on it and be good. There are three or four different types of antifreeze you can put into cars these days, so ensure you are using the correct antifreeze for your application. These are all color coded, so it should be an issue (green, orange, yellow, and possibly red). Each one is specific to certain vehicles and you don't want to mix them. It's not that you'll cause the engine to blow up, it's just that the coolant will not behave like it is supposed to for longevity and cooling effect if they are mixed.

I am not certain I agree with any of the assessment on this link, with the exception of the one that states, if it is consistently low / or empty, you have a problem.

I have a 2003 Nissan Maxima with 196.000 miles on it and recently the reservoir tank has been consistently low, even after refilling it.

I would contend, that if this very same scenario is happening to others, there is indeed a problem. The radiator is a sealed unit, so constantly having to refill any overflow tank is , in and of itself telling you something else is going on. Head Gasket, water Pump, Hoses and a host of other possibilities.

It might be normal. Presure builds up in the cooling system when driving so it can look like it's below L. Best way to check the level of the coolant would be on the cold engine or to carefully let the pressure out of the system buy slowly turning the reservoirs cap. WARNING! If the pressure is in the system and engine is hot it might burn your hands or something else so be really careful and turn the cap really slowly and stop as soon you hear fizzling. after it stops fizzling turn the cap a little bit more and do this little by little till all the pressure is out

I have a Subaru and the expansion in the overflow tank when the car has been running in city traffic is enough to take it from the "cold" mark on the tank to the "hot" mark. That is a fair bit of expansion. The car has to be switched off for 4 or 5 hours for the coolant return to the "low " mark.

I have two cars, both of which the coolant level "assumes" a level well below the min mark.

I have yet to find an explanation on line. If I add coolant, it will escape, I will smell it, and if I ignore it, the level will end up at a lower level and everything is fine in the long run.