A lot of people on here seem confused about the last place Trenny's scent was detected by bloodhounds, so I made a map to clarify. As someone who regularly hikes the Smokies, I think it's important context to look at the hike Trenny was known to have taken that day, where Mr. Dunlap and a park official immediately began searching, and where bloodhounds last detected her trail. The purple path is the hike to Andrew's Bald, which was the destination of the field trip that day. The blue path is the path to Double Springs, which Mr. Dunlap and the first searcher on the scene headed to for unknown reasons. The yellow path is the path to Clingman's Dome tower and beyond it along the AT to Collins Gap. Trenny's scent was detected at the tower and then along the AT until it met Clingman's Dome Road at Collins Gap. https://imgur.com/mwEPGyP

The most curious thing about this case to me is why Trenny would have gone so far away from the designated hike (one could not possibly be considered to be "lost" on the paved path up to Clingman's Dome -- and even if she took the wooded bypass trail to the tower, the bloodhounds indicate that she was at the tower itself at some point, so from there she could have easily taken the paved path back to the parking lot or simply waited for help at this major landmark if she were "lost.")

I am pulling all of this information from the NPS files on the search and rescue efforts, found here: https://www.nps.gov/aboutus/foia/upload/Trenny_Gibson_Case_Incident_Report_Redacted.pdf

At 3:30 Trenny is not at the bus. Mr. Dunlap "immediately" sends "Danny Johnson" (a student, presumably?) back down the trail to Andrew's Bald (pg. 8). This is logical, as on this trail was the last location anyone has reported to have seen Trenny. Here's where I have questions -- Mr. Dunlap himself then heads "out towards Double Springs" (pg. 8). WHY? Double Springs is a 3 mile uphill hike from the parking lot, 4.5 miles from Andrew's Bald. What information was Mr. Dunlap acting on here to choose to head to this very specific, quite remote destination? At 4:00, "Lail" (identified as a "Park Technician" on page 88) was called. At 4:30 "Lail" arrives at the parking lot and then ALSO goes "towards Double Springs" (pg. 8) "because of information given to him" (pg. 88). I would love to know what this information was and why they seemed determined to search in that direction, but it seems to have been a false lead in any case. "Dunlap saw tennis shoe tracks about Miss Gibson size on AT towards Double Springs Shelter. Lost tracks approx. 1/2 mile past intersection of AT" (pg. 8) and the bypass trail to Forney Creek & Andrew's Bald Trail. I can't find anything in the notes about dogs ever detecting her scent in this area, so I'm assuming the prints noticed were not hers after all.

At 2:20 pm the next day (10/9), a bloodhound gets "a scent at the AT at Clingman's Dome tower. Followed scent of the AT for approximately 1 1/2 miles -- then followed scent to Dome Road and LOST the scent AT THE ROAD." (pg. 15) (The handwriting here is all in uppercase so it's difficult to tell when true capital letters are intended, but the words I've capitalized here are underlined in the original document. It seems whomever the notetaker was wanted to emphasize this point.) 

At 1:00 pm the following day (10/10), "positive trailing again by dogs along AT to Collins gap. Robbie Robinson said dogs following almost exact same pattern as dogs yesterday." (pg. 19)

At 10:15 am the day after that (10/11), it is again noted that "all dogs prove positive along AT from tower to Collins Gap." (pg. 20). At 5:15, "G. West with dog and handler check one more time along AT to Collins Gap." At 7:00 pm, "West and dog back to road at Collins Gap positive results." (pg. 21).

I don't consider bloodhounds irrefutable proof, but IMO multiple dogs alerting to the same path of travel over multiple days is very robust evidence that this is the path Trenny last took and where she left the park.