Watermaker feed line stained red?

[got cake]

Disassembled the pipe and fittings that feed the fresh product water from my watermaker to my holding tank. The pipes are all pex and fittings are 316 stainless and plastic with o-ring seals (John Guest). The system has been operating for 9 months, works flawlessly and has probably made around 200,000 liters of water.
I was surprised to find that the inside of the Pex tubing was coated with red with something red. It is solidly adhered to the inside of the pipes in a very thin coating. I replaced the entire feed line but I am wondering where the color came from. Anybody know?

[44'cruisingcat]

Could it be rust? Some "stainless steel" isn't really.

[noelex 77]

Some of the propylene glycol products have a red colour that tends to stain some plastics.

Has a previous owner added antifreeze and left it in the piping?

[got cake]

System was new so not a previous owner but a good lead that I will rethink about everything that has happened with the system with this in mind.
Thanks

[SV THIRD DAY]

It has go be something introduced into the product water AFTER the RO Membrane, because whatever stained your product water line would most likely not makes it past the RO Membrane. How's your product water TDS reading?

[got cake]

I'm pretty certain that it is not rust as I've inspected the stainless fittings and they are all 316 grade and are bright. I think Rich is correct in that it must be introduced after the membranes. TDS is great at around 120 and the water is excellent taste and odorless. It has never been pickled or chemically cleaned.
Next step for me is to disassmble the four threaded fittings and see what was used to seal the threads

[Tellie]

Actually I think noelex 77 is the closet if not on the money with his answer. All due respect to my friend Rich, as Noelex77 stated, some PG use dyes to identify their concentration and many times other additives and this dye certainly can get by a SW membrane. I have been experimenting with PG concentrations for storing watermakers for a few years now. When we realized that PG was a viable pickling agent we went for the higher concentrated PG. More is always better right? Getting the right concentration and mixture between the PG and the existing fresh water in the watermaker itself has been a bit of trial and error and a long story in itself. But what is usually missed is when the watermaker is first shut down and the pressures dissipate within the membrane itself the membrane tends to, for a lack of a better word, absorb some of the media that surrounds it inside the membrane vessel, be it salt or PG. So when the watermaker is fired up again for it's next use the amount absorbed will migrate to the product side of the system. If you pulled your membrane to inspect the inner product tube, my guess would be that you's see some of this same red staining. That being said, I hope that the color is really more pink than red. Red could be a warning sign that the wrong chemical is in play. Ethylene Glycol can be colored red and EG can be deadly. Also, not all PGs are the same and not all are suitable for watermakers as many contain cleaning and protectant agents that do not play well with watermaker systems. It's always best to check with your watermaker manufacturer or tech to ensure you are using the right PG and in the right concentrations.

HaldenMarineService@yahoo.com
(954) 515-7077 l

[got cake]

Thank Tellie, most of what I know about watermakers I learned from reading your posts. I have tried to attached a picture of the offending pipe.
I am now trying to determine if it might be loctite and also, what might have happened with the system when it was not in my control. Maybe it did get pickled before it was delivered to me.