the battery just keeps dying out on nissan micra?

i just recently purchased a nissan micra 03, petrol but the battery just kept dying flat so i replaced it with a new one, after a couple of days the new one died out too,changed it again and also had the alternator checked out everything seems fine but just cannot figure out what is wrong, have checked if any lights remain on while locked but no luck too. have changed the battery again 3rd time after replacing a new one it works fine and couple of days later it dies out again,please help