Keeping your laptop open allows dust to get into the speakers easier as well if they are the type built around the keyboard.
My laptop is always open. Will it be dirty or dust keep coming in? I wipe it twice a week.
I have the same experience and the dusty part would be at the back of your laptop or the covering. Make sure to clean them too.
Yeah I just noticed awhile ago. You are right. I should wipe it often.