The following code draws a circle at the center of the page, ignoring the "shifted" part.

\startuniqueMPgraphic{concept}
draw fullcircle scaled 1cm shifted (10cm,10cm);
\stopuniqueMPgraphic
\defineoverlay[concept][\uniqueMPgraphic{concept}]
\setupbackgrounds[page][background=concept]
\starttext
.
\stoptext

How should the metapost background figure be positioned correctly?

Thanks!

Best,

Sylvain