[NTG-context] MetaPost visibly drawing at 0pt

I used to put an invisible frame around figures in MetaPost by setting a pen scaled at 0pt. That left no trace, but defined the area. However I (now) see a thin line drawn by code: pickup pencircle scaled 0pt; draw origin--(w,h); Did I always mis this or has something changed? For example a roundoff away from zero in MetaPost? By the way I am typesetting through ConTeXT with \startMPpage … \stopMPpage. Hans van der Meer