You flew over the Outer Marker. There are three (Outer, Middle, and Inner) on many, if not most approaches. They’ll be indicated by O, M, and I visual indicators, along with audible tones. They’re useful in situations with limited visibility so that you know how close you are to landing. You can even be flying at cruise altitude and once in a while fly over one and get the visual and audio indicator. If you look down real quick, you’ll see that marker’s airport below you.
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