Originally Posted by shanes teeth
It points to your house's "key brick". If this brick is removed or damaged it will cause the house to fall down.They are built in to houses to aid future demolition.
This is total nonsence, so don't worry.
The stone with an upwards arrow (/|\) with a line across the top, is known as a BENCHMARK, or official name ORDNANCE DATUM, which is often marked with the height in feet and inches above Mean Sea Level, and is an aid to surveyors in surveying streets and property. You will find them located frequently around towns, also in many places in the countryside, in particular churches, on stones on hilltops, and on official buildings. As a former Boy Scout, a keen youth hosteller, and an archaeology graduate, I also am familiar with their importance in helping with map-reading, as they are good location markers; archaeological digs have to be mapped to a high degree of accuracy, but it is of fundamental importance that they are tied into the National Grid of references by being triangulated to an Ordnance Datum, although Satellite/GPS instruments can now supersede the use of a fixed point on the ground.