I think both are different issues, because the objects in the scene of Matthias are helpers of Particle Flow (Forest does not use P.Flow in any way).
Matthias, have you used Particle Flow as a scattering tool to distribute archmodels objects ? These helpers would come from here...
Pierre, if possible please send us a scene with the problem that you describe. The only point where Forest holds external objects, is in the layer "forest_templates" (when a model is loaded from the library). But this is a standard layer with the hidden flag on... if this is not the problem, i would like to investigate it.