Rust in trans from dip stick tube?

Getting ready to go on a trip, I was checking the transmission fluid in our 06 E450 and saw some chunks of dark material on the last 2" of the dip stick. I felt it with my fingers and it was like a fine dirty rust. I cleaned off the stick put it back in and got only one small lump of it, but the fluid itself looked and smelled fine.

I took it to a local Express Lube to have the trans flushed, but they didn't have the correct adapters so could not do it. The owner of the shop said it is most likely from rust in the dip stick tube from sitting long periods. Which is quite possible since the rig is nearly 10 years old and has only 13k miles on it. We have only had it a little over a year and bought it from the original owner. But I make a habit of taking it out once a month or so to keep everything moving in it.

It is too late to schedule a regular shop to do much about it right now since we are leaving in the morning. But when I get back from our trip, I will be ordering a new dip stick tube and have the trans serviced with a new filter. Just curious if anyone else has had this kind of problem and if in fact it was caused by the dip stick.