Gravesy would not tell. Issy is not knowing Gravesy but a good house elf would not be telling its house's secrets and Issy is sure Miss Luna must have a very good house-elf for Oddswallow Hollow.

Sometimes cleverer wizards and witches is knowing when their houses is happy or is sad. Issy is not telling any secrets in saying Miss Luna could just be listening very carefully to is the house happy or sad.

Issy wishes she could be mroe helpful but she is a stupid eld and does not know how. Oddswallow Hollow is not Issy's house, and if it was being Issy's house then Issy could not be talking about its secrets. Issy is so sorry she will not look at any buttons all day.