Petfon's performance could not be guaranteed in woods,forest or tunnels. In order to explain it more clearly, please kindly allow us introduce the working mechanism for Petfon.
Petfon is positioned directly by GPS, while GPS signal could be blocked by obstructions,  the obstacles could result into a weak positioning or even positioning failure. Please kindly check the official GPS website in below for a better understanding, in their website, they clearly explains how the GPS technology works, not only for Petfon, but also for any other device that applies GPS technology.
https://www.gps.gov/systems/gps/performance/accuracy/
In our case, the positioning of our tracker depends highly on the obstacles between GPS satellite and the tracker, if your pets run into some areas with heavy shelters above---Forest------GPS would not be able to position it accordingly.