You log it as "unknown community transmission". While you obviously can't trace everyone, California has 10,000 contact tracing staff [1] and has spent most of the pandemic with less than 5,000 confirmed cases per day [2]. These individuals are working every day to collect data, so I am sure it exists.

If you get a positive test, contact tracers may ask you where you went and who you interacted with. They then call those people and ask them to get a covid test. IF the contact tests positive, then rinse and repeat. [3]

If I live alone and only leave my house to grocery shop, it is pretty clear where I got covid. Similarly, if Bob had covid, I had dinner with Bob, and now I have covid, it is pretty safe to assume I got at dinner.

Anecdotally, I had a friend who tested positive and the contact tracer told them that they had likely contracted while it shopping at a big 5 several days earlier.

https://coronavirus.jhu.edu/contact-tracing/state-survey-res...

https://91-divoc.com/pages/covid-visualization/states.html

https://covid19.ca.gov/contact-tracing/