Found it! Sun did find it:
Charlie wore the ring for the 91 days he survived on the island. However, before leaving for his fatal mission to
the Looking Glass, Charlie left the ring in
Aaron Littleton's crib, passing the ring on to the infant who in their short time together on
the Island had become something of an adopted son to Charlie.
("Greatest Hits")
Sun finds Charlie's ring, three years after
his death.
("The Incident, Part 1")
However, Aaron never received the ring, as he never returned to his crib before leaving the Island as one of the
Oceanic Six.
("There's No Place Like Home, Part 2") The ring remained in Aaron's crib during the time flashes.
When encountering the camp in 2007, on the way to see Jacob,
Sun finds the abandoned crib and the ring within it