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They are basically a one way diaphragm valve that lets air in the drain line so the water flows to the tanks and not burble like when you pour a milk jug. There is one after the p-trap on all three of my sink/shower lines. It is a pvc cap looking thing that screws into the threaded pipe and they are higher up than the p-trap so as not to let water out even if they were missing. If you follow the drain from the sink, past the p-trap and before it goes down to the tank, it should be in line if you have them. They vent the lines without having to continue a vent pipe to the roof for each drain. Like in house construction, you need to let air into the drain system to make the water drain to the tanks efficiently.
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