I learned that from one of the TSA line prep barkers. They were putting such a jolly spin on their spiel and it sounded so ridiculous that I assumed it was a joke. But, both happily and sadly, nope.
They're growing in popularity.
I did once accidentally leave a large, full bottle of contact lens solution in my carry on, though, and it passed right through. Maybe that's the trick.
You could probably claim contact lens solution as a "medically necessary liquid," which is exempt from the rules anyway.
When I do this, they often will spray a bit of the solution and test it for something or other.
That's not a loophole that could be exploited. /s
On a side note, could one seek to get water treated as a medically necessary liquid? It is, and the rules say nothing about being limited to only counting medically necessary liquids that cannot otherwise be obtained on the other side of security.
Far less. I, too, never remove this stuff and I've never been asked to.
The detection rate on a flask with vodka in it is slightly higher, which makes me concerned as to what other things you could bring with you as long as it's in an xray-opaque container.
I travel with an empty water bottle every time.
Half the time, the TSA misses my solution bottle. The other half, they find it, have me open it, wave it under a chemical detector, and then send me on my way.