I just surly detectived this personally recently
The Grotto is a very busy shuttle serviced stop with multiple popular trails in multiple directions. It is the busiest stop in the park for hikers
The one she was found near is close to the road.
Shuttles drive up and down the road all day.
In a canyon, sound travels. You can clearly hear the shuttles, and people, at a distance.
Typically Zion is packed with hikers. Last year over 4 million people visited the park.
Unless you are a goat, you’re not hiking out of the canyon. The river and the road run right down the middle. You can’t not see where you are with 2000 foot cliffs above you. Between them, at the lowest point, is the river, and the river is next to the road. You can’t be at the river and not see the road.
Signs all over warn you about the river.
And finally, there is a water station at the Grotto, I used it.
Same for the Lodge stop just to the south, except there’s actually a hotel there.
You could walk from one stop to the next in about 10 minutes.
No way could you consider where she was as off trail or backcountry. This is high desert scrub, not a jungle in ‘Nam. It ain’t that hard to see.