My 12 y/o ds the eldest of my D.C. has clearly just found his favourite toy. 
That's fine but my 6yo Dd told my oh that she found him "with his hands in his pants"
My DJ found him x 2 and also the conv with dd and explained to ds that it's entirely natural to do this but he needs to do it in private
Except I have found him twice in bed with his door open (his bed is clearly visible to anyone going las) doing this. Once was quite late in the evening so his younger sibling were in bed asleep but today it was 3pm in broad daylight.
Both times I have not said anything, been too shocked and have to go past his room and so have just carried on rather than "interrupt" but I think we have to say something
Any advice?