Follow the Visibility rules for normal scenarios. Note that using light sources to illuminate dark regions usually results in immediate defeat if a Seeker can see the light (from within the region or an adjacent unblocked region, for example). Individual scenarios will specify any particular rules for this.
Many stealth scenarios also have unique rules to limit activities that would automatically result in the PCs getting caught. For instance, defeating doors through EP (bashing them down) may be disallowed, with lockpicks or keys being the only option for bypassing such Obstacles. The DM should announce any special rules for a given scenario.
For more information, see Player's Guide Chapter 5: Exploration.