Apotropaic marks and objects are often found in old houses. They were intended to turn away harm by distracting malevolent spirits. Examples include inscribed circles, sometimes with a daisy-wheel in them; double Vs (for Virgin of Virgins), random-looking scratches, and sometimes even runes. They are not a pagan custom, though they do indicate a belief in magic. Objects that have been found include dead cats (one hopes they had died of natural causes before being placed in the wall cavity), shoes, bottles and pots.
There are some apotropaic daisy-wheel marks in the doorway of the tithe-barn at Bradford-on-Avon.
Such marks are related to, but not the same as, mason's marks (which often had magical associations but were not put there to turn away harm).
Contemporary Pagans sometimes make spirit bottles (bottles containing coloured threads). Of course you can also carve or paint runes or other marks in your own house but you should never ever do it on old buildings or ancient monuments (and 99.99% of Pagans would never do this anyway).
There is a lot of information on the use of spirit patterns and objects in Nigel Pennick's excellent book, Practical Magic in the Northern Tradition.