Hello I have a recurring problem with some pest. Not sure if what I am seeing is the pest itself or evidence of it. Always occurs under windows - sometimes found on floor, woodwork or windowsill. Windows face south west. Please see photos. Thank you !
Based on your photos the debris looks sawdust like. We cannot say what insect is making the debris. You did not mention if the house is new or older. If the house is new it is possible there may be powder post beetles in the wood. Also, Check for blistering paint, signs of moisture, leaks, and any wet wood in the area. Look for small ants in the area. Ants can take advantage of this type of area for nesting.
Clean up the debris and look for holes outside, around and under the windowsills. Put tape on the bottom of the windowsill to try and figure what is coming out.
If the area has a leak and there is rot you will have to replace the wood.