The setupexternalfigures seems to be broken. In the example below, the first \placefigure doesnt work, but the second with the entire path does.
% mptest.tex: \starttext \setupcolors [state=start] \setupexternalfigures [directory={fig}] \placefigure [top] [fig:three-point-setup] {This is the caption.} {\externalfigure[figures.1]}
\placefigure [top] [fig:three-point-setup] {This is the caption.} {\externalfigure[fig/figures.1]}\stoptext % fig/figures.mp: beginfig(1) pickup pencircle scaled 5bp; draw (0,0) withcolor (1,0,0); endfig; end