There are two factors that relate to your question. 1) cutoff for the test (usually 100 or 500). 2) time of consumption in relation to time of test.
Generally, incidental exposure are in the lower range 100-300 ng. A 100 cutoff greatly increases detection due to incidental exposure and consumption of products containing small amounts of ethanol... and there are thousands of possibilities for such exposure.
There have been several studies involving trace amounts of alcohol in consumables and personal care products. Generally, you must be careful during the 12 hours leading up to the test. That is because EtG can spike above the level of detection but is eliminated with a few hours. See Musshoff, 2010
The estimated EtOH content of Kombucha is the same as non-alcoholic beer 0.5% and there are several studies involving NA beer. Ex. Thierauf, 2010
The above is meant simply to provide you with a starting point in researching the EtG test. It is not a system that provides protection against false EtG results.
EtG testing is erratic, unreliable, absent a scientific foundation, inadequately researched, widely misrepresented and should not be in commercial use. Why EtG is still used by some professional licensing boards is incomprehensible.
Here is link to a discussion group involving nurses and EtG testing:
Ethylglucuronide
Musshoff, et al 2010
Ethyl glucuronide and ethyl sulfate in urine after consumption of various beverages and foods—misleading results?
Thierauf, et al 2010
Urine tested positive for ethyl glucuronide and ethyl sulphate after the consumption of “non-alcoholic” beer
NOTICE: Any person or entity that is subjected to or relies on EtG testing should read this Advisory issued by the US Department of Health and Human Services, September 2006 and remains in effect as of date of this posting. Copy available here >
US DHHS Advisory