Silicone Radiator Hoses
I guess everyone else already knew but I wish I knew about this sooner because it's been driving me crazy ever since I rebuilt my engine. Silicone hoses indeed are extremely permeable and you lose a lot of coolant to evaporation through them.
Every time I stopped my car after it was hot it would reek of coolant for hours, and I would find the level in the reservoir dropping significantly even after the engine cooled and the car sat for a couple weeks in the garage. Couldn't find any leaks and it was driving me crazy.
I had put silicone hoses when I upgraded my radiator because the rubber ones were old, and the silicone ones were a lot easier to get on the ports. Now I put new OEM hoses on (with new OEM spring clamps) even though it was a giant pain in the ass to get them to stretch on my CSF radiator. Car doesn't reek like coolant all the time and the reservoir level is steady now, sweet!
I was using Grimmspeed silicone hoses, but I suspect the brand doesn't matter and this happens with all silicone hoses.