I don't understand the risk of scanning them?
The potential risks, as I see them, are:
(1) website spoofing,
(2) theft of personal information from the phone, and
(3) activation/monitoring of phone's GPS signal.
If a person was so inclined, he could print a QR code of his own and affix it over the official QR code on the signs. This false QR code could open a malicious website (that spoofs the official website) and while downloading confidential data from the hiker's phone also alert the criminal that someone has just scanned his code. This would then allow him to begin tracking the phone's GPS signal. These signs, remember, are located on trails in semi-remote areas. With this setup, the criminal could execute any number of plans - all of which would end up badly for the poor hiker. Just as the vibration on a spider's web alerts the spider that dinner as arrived, this QR code-generated alert would let a predator know exactly where is next victim is located.
The beauty of this setup (beauty from the PREDATOR'S point of view) is that the actual crime (rape, robbery, etc.) could occur far from the actual trail sign. The victims, if they survive, would have no idea why they were targeted. And LEOs and investigators might never make the connection between the signs and the crimes.
Possible? Yes. Probable? I don't know - depends on how motivated the bad guy is.
And if any of you turn this idea into the next bestselling crime novel, I would appreciate a small royalty.