Courtney Bryan’s most recent Instagram post shows her relaxing in the Hunt Hot Springs at the Shasta-Trinity National Forest, northern California on 23 September.
The 32-year-old is believed to have spent the day at the secluded spot 270 miles north of San Francisco with her dog and travelling companion, a black-and-white border collie mix called Booch.
After sending a text message from near the springs just after 9pm that night, Ms Bryan vanished, along with Booch and the maroon 2004 Subaru Legacy they were travelling in.
Desperate for answers and having seen how effectively social media was used in the Gabby Petito case, Ms Bryan’s sister Katana Curven has taken to TikTok asking for help.
“Hey TikTok, I need you guys to do your thing, please. My sister is missing, her name is Courtney Bryan,” an emotional Ms Curven said in the video, which has attracted more than 650,000 views in the past six days.
“No calls or texts have been sent from her phone since then,” she says on the clip.
Since her first TikTok six days ago, Ms Curven has posted four more videos appealing for help, with practical information such as maps where her sister was last seen, reports of possible sightings and the hashtag #findcourtneybryan.
She said her sister is an avid hiker and climber, whose “whole world” is her nephew, and that the disappearance was totally out of character.
“She’s a free spirit and would often travel alone,” Ms Curven told Dateline.
“But she would’ve let us know when she was coming back. She would’ve checked in with somebody.”
Ms Bryan’s grandfather had died on 16 September, and she had taken a few days off from her job at outdoor apparel store Patagonia in Reno.
When she didn’t return to work on 27 September, her family reported her missing.
“It makes sense that she wanted to get away,” Ms Curven said. “But she would’ve been back by now. She wouldn’t let us worry.”
Ms Curven added that her sister would never have left her cat and plants without asking someone to take care of them.
The family have traced her phone records and found her final calls and texts were made on 23 September from the Shasta-Trinity National Forest.
Ms Curven told Dateline she had called a former boyfriend seeking directions to the hot springs where she took her final selfie.
Ms Curven said she had been in regular contact with authorities and was being kept up to date on possible sightings, but none have turned out to be her sister.
Ms Bryan is described as being 5ft7inch (1.7m) tall and weighs about 120 lbs (54 kgs), with blonde hair and blue eyes. She has two tattoos, a star on the inside of her right wrist and a geometric honeycomb on her calf.
Anyone with information on Ms Bryan’s whereabouts should contact the Reno Police Department Missing Persons Unit at 775-321-8372.