From an earlier post of mine on the same subject:
Venting is how all car makers deal with the pressure gradients that exist within modern headlight housings. The space inside headlight housings experiences a large temperature fluctuation depending on weather conditions and whether the lamps are on or off. As we all learned in high school physics (yes you did), when air heats up within a confined space, it expands & the air pressure inside the space increases. Given today's designs molded in plastic with large exterior surface areas, it is necessary to prevent either a positive or negative pressure situation developing inside the flexible plastic enclosure to prevent flexing stress to the plastic exterior walls. For this reason, car makers use a vented housing design. Most car makers uses simple vent holes covered with a semi-permeable membrane. The membrane's purpose is to keep out liquid water, dust particles & mold spores, but allow moisture-laden air to flow freely across it in order to balance internal air pressure with ambient air pressure once the lamp element (or sunlight) begins heating up the interior of the housing.
The consequence of using a vented design is that sometimes condensation may form on the inside surface of the headlight lens, depending on the ambient relative humidity & temperature. This condensation is driven out as the lamp element (or the sunlight) heats up the housing interior and the moisture-laden air is vented to the exterior. Depending on where you live, or the season of the year, you may not see condensation form on the lens. If you live in a colder, damp climate you probably will. In a desert climate, probably not.
Now, if there is liquid water pooling in the bottom of the housing, very likely you have a lens seal breakdown & that will require housing replacement. GM will replace these units under the bumper-to-bumper warranty.
Some dealers are more lenient than others regarding housing replacement for condensation. All you can do is ask. But don't be too surprised if they do replace them, that the replacements act similiarly.