In most airports check-in area is not a secured area and you can enter it and leave it as you wish, no matter whether you have a boarding pass or luggage or whether you plan to fly or not. It's just a big hall behind the entrance doors.
So if Mumbai airport doesn't have some peculiar plan, you can leave it and come back later at any time.
Note: do not pass security control after you check in your luggage. You' won't be allowed to leave the security area after that.