The exact same happened to my Alto after a long trip, it kept happening every morning I checked, I kept topping it off but it kept going down to the same level. I got the system bleeded, as it also had some air trapped, Changing the radiator cap, and looking for any leaks is what every forum on internet recommended. And bleeding the system helped me. I also asked a few mechanics about it they said if the reservoir is filled there shouldn't be any issue.
But please do your research.
