Dark lines on ceiling

I've noticed for some time that certain times of the day, I can see faint darker lines on the ceiling. They almost look like shadows. They follow the lines of the aluminum rafters. Also, I see it on some of the interior walls as well. Is this normal?

Do you burn incense or candles inside? The residual smoke will outline the structural components moreso than the rest of interior walls and ceiling.

You were right! We do burn an occasional candle...and cooking smoke perhaps. Used a mild detergent, and it cleaned up fine...it's just intersting that it outlined the framing members. thanks for the advice!