Heraldic graffiti is not uncommon, although sadly it is rare for it to be positively identified with a particular family.
Heraldry relies on colour for identification purposes, and graffiti inscriptions contain no colour (or if they did, it has been lost). So even a neatly executed shield, with identifiable elements, but no colour, could potentially have belonged to several different families.
Heraldic graffiti can vary enormously in detail and quality, from full coats of arms, to simple scratched outlines of shields, seemingly no more than generic.
It is notable that this type of graffiti appears the most likely to be subsequently defaced. It seems possible that this was due to malice against the individual or family represented.