Amelia Bambridge, 21, from Worthing, West Sussex, was last seen on the island of Koh Rong late on Wednesday night.
Staff at Police Beach, a private venue that stages regular events, found Ms Bambridge's purple rucksack, containing her purse, phone and bank cards, the following morning.
During a phone call with her younger sister, Georgie, on Wednesday, Amelia said her first solo trip was "doing so much for her confidence" and she was having "the best time ever".
Georgie Bambridge told Sky News: "She said 'everyone is so cool here' and couldn't be happier.
"She's literally wanted to go travelling her whole life.
"She has spent the past two years saving for it and spent the last year reading blogs of travellers, researching what she was going to do.
"She knew everything and was living out her dream - she was happy."
Ms Bambridge said her "role model" sister "would never drink to excess" and was "always in control", adding: "She would always be looking out for me, it's not like her to not look after herself.
"I just don't know what's happened."
Amelia set off on her trip on September 27, when she flew to Vietnam, her family said.
They were alerted to her disappearance after she failed to check out of the Nest Beach Club hostel.
Her passport remained at the hostel.
Local volunteers and Cambodian police have scoured the shoreline and jungle for the missing backpacker, and Ms Bambridge's family are also heading to Koh Rong to assist.
Her father and brother have already arrived on the island, and brother Harry wrote on Facebook on Sunday: "So I've landed in Cambodia, in a taxi to attempt to make the last ferry to the island, which is a five hour taxi.
"I'm totally overwhelmed with everyone's support and I can't thank you all enough."
In a different post, he added: "I seriously believe there is still hope for her to be found alive."