>>8651541 (pb)
Construction/Architectfag here.
Thank you for posting the pics.
Assuming there is an innocent explanation, I would guess these marks were made by the carpenter who installed those trim pieces. (They marked the X on the garage jam and aligned the X on the trim piece.) The reason for doing this would be so that when you install the overhead piece, it aligns with the side pieces and remains level.
So my question(s) would be, have you noticed these before? (Have they always been there, and you just didn't notice?) How long has the garage trim been installed? Can you rub the X off with you fingers?
I'm skeptical that this is somebody "marking" your house. (Though it could be.) Mainly because it seems way too obvious, and why make multiple marks? It's not as if these would be clearly visible from the street, and there are far more subtle/effective ways to do this without drawing X's in permanent marker.
Just my 2 cents. Best advice would be to check with your neighbors to see if they have anything similar, and go out and buy a Deer Cam from your local Bass Pro. (I did this before when my wife had a stalker. Very easy to set up and check.)
Good luck, and let us know what happens.