Fog and other low-altitude moisture often shows as very light precipitation radar returns. I look at the multi-image radar loops for an area, and if I see that I call a "sparkling" radar return in the looping image, it's a pretty good bet for ground fog. The California central valley area (around Modesto and Stockton) is known for its winter tule fog: watch the radar returns from that area in the wintertime.
You can correlate the radar observations with ground visibility observations (airport METARs, the regular and special airport weather observations, include visibility and ceiling—cloud height and coverage—information), along with satellite visible and infrared images to view cloud presence and cloud top heights. When the visible satellite shows clouds and the infrared satellite shows very low cloud tops, that's a good bet for fog or a low-level stratus layer: both have low cloud tops, typically 400'-1,000' above ground level.