Kombucha and ETG

From what I've read about Kombucha some products have been pulled from the market due to alcohol content in excess of 0.5%. The alcohol isn't additive but rather results from in vitro fermentation post-production. That is a natural occurrence in the presence of yeast and sugars. Some of these proprietary products can range up to 3% alcohol. Media example

There are two factors that could create alcohol (EtOH) in the process you describe in brewing your own. The first is fermentation in the container and the second is fermentation in your intestines (in vivo) after consumption or a combination of the two. If EtOH is present then EtG/EtS will result. It's simply a matter of how much of each.

Addressing the in vivo fermentation. This phenomenon is well documented. The presence of yeast and sugars in your diet can lead to production of EtOH within your body. This occurs primarily in the intestinal tract where exposed to various microbes (Auto-brewery syndrome).

Another weakness of EtG testing, acknowledged in professional journals but not otherwise publicized. (see Skipper)

In specific reference to EtG and EtS a 2010 study found that abstinent volunteers consuming sugar and baker's yeast orally produced elevated EtG/EtS levels above the cutoff of 100 ng. (see Thierauf)

Ref.

Thierauf, et al, 2010

Skipper