Unless you're very careful I think it should be detectable at those levels. If someone is taking the same amount of time when playing some rare and tricky opening line as they are when playing the QGD, that's pretty fishy if they don't have the level where you have a big variety memorized.

> It's pretty helpful for the average human. Most people can't blast out 15 moves of theory correctly for every possible opening, so you'd avoid a lot of bad middle games or losses to traps.

Yeah, although at that level lots of people are also getting into good positions, having no idea why it is good, and losing it.