On more than a few occasions I have noticed the WX radar sweeping along merrilly and displaying a nice solid bank of buildings in front of us.
Not sure what a/c you are talking about but it's more than likely the Enhanced groung proximity warning. It paints on the screen using the same system as the weather radar and can be confused for it, to avoid confusion on Airbus at any rate, it sweeps from the centre outwards rather than side to side. Unlike the radar there is no transmission taking place. Ask the crew next time and I am sure they will explain it to you (or switch the radar off in an embarrassed hurry if you are correct!)