If the yellow warning appears intermittently, something is momentarily blocking the Time-of-Flight (ToF) sensors. For example, fingers covering the ToF sensors at the front, or clothing hanging over the ToF sensors near the front chassis.
If the yellow warning appears permanently, the sensor surfaces are dirty and due for cleaning.
There are 10 ToF sensors in total. Eight are shown in the image above, and the remaining two are located behind, on the left and right of the rear camera.