Muir survived the glassing of New Llanelli, and survived on his own on the planet for seven years until he was found by the crew of the UNSC Corvette Port Stanley. He witnessed the Sangheili and Kilo-Five trading weapons, and was spotted by the ODST Malcom Geffen in the process.
He was taken on board the UNSC Port Stanley and kept in quarantine in order ensure he did not see anything regarding Kilo-Five's operation.
1. Halo Glasslands, page 131, 132, 133