An international network of/for intelligent organisms
That is a good question, one that I have yet to see addressed anywhere since I have been studying slime molds or may be there was and I just missed it. However, I have noticed that everywhere P. polycephalum goes, it leaves behind a noticeable debris field along with the slime track of course. The debris field, the make up of which, is also evident being scattered about in the plasmodium before hand.
The white trail is pretty much it. If you feed them a little powdered charcoal in their oats, you'll see how they pick it up and excrete it in their slime trail. They'll do the same trick with some fabric dyes which leads to some interesting art possibilities...
Didn't know that, I've got to give it a try.