Star Trek: 10 Classified Starfleet Secrets

In the Next Generation episode Clues, the Enterprise-D inadvertently stumbled into the home system of the Paxans, but everyone, other than Data, had the memory of their encounter erased.

The Paxans are a highly xenophobic species. They hate outsiders so much that they conceal their homeworld with an energy field that stuns the crew of approaching alien ships, rendering them unconscious until they leave the system. The field also tricks sensors into detecting a fake wormhole so when the crew awakens, they assume that the journey through the wormhole was the cause of their unconsciousness and spontaneous jumping across lightyears.

Unfortunately for the Paxans, Data, as an android, was immune to the effects of the energy field. At first, Data was able to revive the crew, but in order to escape certain destruction and appease the Paxans, Picard had to allow them to erase the crew's memories. He ordered Data to never reveal the existence of the Paxans, but clues were left behind that led Picard to uncover Data's secret. The Paxans agreed to give them a second chance and once again erased their memories, this time taking special care to remove any residual clues.

Data presumably kept the existence of the Paxans secret for the rest of his life.