Great Western Coffee Shop

Bus windows are made of some kind of safety glass.  The details of the glass type and manufacturer is typically printed in a comer of the pane. 

For many years now, I have noticed that this information is often circled or drawn around with a marker pen or chinagraph pencil.  It is usually done fairly neatly but freehand, and doesn't appear to be limited any particular bus company. 

I would like to know if anyone else has spotted this and if anyone knows why it is done.  I have two theories...

1, it is vandalism.  But it looks like it is done too diligently and neatly for that.
2, if is done as part of a safety inspection carried out on the vehicle perhaps as part of an MOT test or similar and someone is charged with checking that each pane of glass is in good condition and bears the appropriate indication that shows it is the right kind of safety glass.  But why would the inspector then outline the information in marker pen?  It is not like it is difficult to spot or tick off without marking the window and I can't quite believe that it is sensible for a bus company to have buses driving round with marker pen defacing the windows.  Even if it is done fairly neatly, it is an encouragement to vandals to also deface the vehicle is it not?  And why is it only done on bus windows.  When I get my car back from its MOT, I don't expect to find the safety-glass legend circled, any more than the I expect to find my tyres chalked to indicate that the tread has been inspected.   

 

Isn't it done for the Certificate of Fitness exam? I presume it's an easy visual proof that every window has been checked.

Yes. I've seen this question somewhere else a few years ago and this was the agreed answer. Its circled so that the inspectors/examiners can easily locate it. It's not always in the same position in every pane. Could be bottom left / right, top etc especially if the glass has been replaced at some point.

Thanks.  Still seems an odd thing to do.  It isn't like it is a difficult thing to locate and "official" pen marks on the window might act as encouragement to unofficial ones.