The dampening field over Vulcan's Forge, which extended "only a few hundred meters above the ground," was capable of preventing the sensors of the patrol craft flying over it to detect what was inside.
After some time in the Forge, in 2154, when T'Pol and Jonathan Archer were being sought by those supporting Administrator V'Las, T'Pol noted that as they approached the T'Karath Sanctuary, that "the patrol craft are flying again. They sound like they're closing in on this location." (ENT: "The Forge")
After they reached the sanctuary, and word of the death of Syrran along with them, T'Pau noted that "search patrols are flying day and night," however, T'Les felt that now with the death of Syrran, the patrols were "the least of our worries now."
At the same time, based on the data collected by these search patrols during the missions these craft had been conducting over a period of several days, V'Las was able to pinpoint the Syrranites location, near the northeastern border of the Forge. (ENT: "Awakening")