The picture below shows the most common locations for serial numbers. Pianos often have numbers other than serial numbers . (Such as part numbers, series numbers etc.) It sometimes helps to use a flashlight when looking for the serial number. The big S indicates where to look.
Grand Serial Locations
Picture courtesy of Pierce Piano Atlas
If you cannot find the serial number in any of the normal places, check underneath the piano.
If that doesn’t work, it is possible it is inside near the action.