There is a palace in Scotland where there is a stain on the floor. Which palace, where is it and what is the stain purported to be?

Some of Scotland's bloodiest history has taken place within the ancient walls of Holyrood Palace.

This is where David Rizzio, the secretary of Mary Queen of Scots, was murdered before her very eyes in 1566. Until recently the 'blood' stain on the floor was repainted every year as a tourist attraction.

https://bumpinthenightblog.wordpress.com/2012/06/20/nearly-royal-wraith-palace-holyroodhouse/

I was there in 2002 and the "blood stain" was still there. Edinburgh.

There was a lovely case in a church somewhere in Europe where the blood of a saint appeared regularly - still wet - on the floor. British feller (Beckford? some name like that but there's two similar ones and I get mixed up), who had eaten almost everything eatable or tastable, dipped his finger in it, tasted it and declared it to be bat's urine. He was speaking from experience.

Holyrood House the blood of Rizzio servant to Mary Queen of Scots