Disclaimer – JKR owns the rights to the Harry Potter universe. Thanks to author Arekay for giving me this idea on the use of the Fidelius Charm.
Lily Potter was both scared and angry. For nearly two years, she and her husband James and later their son Harry, had been running and hiding from a terrorist group called the Death Eaters, due to a prophecy that their son would defeat the leader of the gang. Her one year old son, who could hardly pronounce ten words and had only recently become coordinated enough to walk on his own.
She thought about the situation for a few days and decided to contact her former Professor of Charms to try to find a charm that will help her hide. Two weeks later she was told about the Fidelius Charm. She spent a further week learning how to perform the Charm as well applications for its use. One night she was listening to her husbad and his friend Sirius Black talk about pranks they had played and she had an epiphany.
As there was a rumoured spy in the Order she decided to keep her experiments to herself and that night she put the unforgivable curses and the prophecy under the Fidelius. She was laughing quetly at the next meeting of the Order of the Phoenix, an anti Death Eater organisation. The Death Eaters had suddenly become less effective.
Her next move was hiding the identity of the top DE financiers, as well as their Gringott vaults. She made sure that she was in the bank the next day. She had to cast a notice me not on herself to prevent the DE's from cursing her for laughing at their misfortune. They could not tell the goblins who they were and the goblins did not recognise them and were therefore denying that they had vaults. Even blood tests were not proving their identity. She made sure to tell her husband James that since they could not prove they were pureblood, they were not subject to the purebblood exemption from the truth serum veritaserm. Questioned on their criminal activities, they revealed their many misdeeds.
More interesting however was the news that was revealed at the next Order meeting. When the DE's met, a number of them were killed as spies as no one recognised from her experience, Lily hid the identities of small groups of DE's each week. The DE's size was reduced each meeting.
Her last use of the fidelus was to hide the existence of the the pureblood belief. As the DE collapsed as they could not remember why they were meeting, Lily relaxed, her son was safe.