Arguably, most of the party-goers were out-of-state, but it also would've been relatively easy for authorities to link a spike in deaths to those events. So far we have not seen the spike take place.
What makes you say it "would've been relatively easy" given contact tracing is not widespread in the US and certainly wasn't during the weeks of US Spring Break (ca. mid-Mar)?
In any case, you are mistaken that "we have not seen the spike take place". [0] See, especially, the map "generated by Tectonix GEO and X-Mode Social" about 1/3 of the way down the page.
[0] https://www.nytimes.com/2020/04/11/us/florida-spring-break-c...
Plane tickets were bought, credit card spending took place, cell phone towers were pinged, hotel reservations had names linked to them, relatives, friends or colleagues aware of the whereabouts and can be interviewed post-mortem?
For what it's worth, they're able to pinpoint specific parties or night club performances https://www.pasadenastarnews.com/2020/05/09/pasadena-identif... https://ktla.com/news/local-news/former-rancho-palos-verdes-...
https://www.forbes.com/sites/rachelsandler/2020/05/13/elon-m...
Keep in mind that the young person might be a medical worker, or in the family of a medical worker.
There were 58 deaths reported by 3/14. If you assume that mortality is 1% (probably on the high side, but good enough for estimates), that translates to ~6k cases.
However, since we know that deaths lag cases by 1-3 weeks (let's assume 2 weeks for the analysis), we should be looking at deaths two weeks from 3/14, which would be 3/28.
By 3/28, there were ~3k deaths from COVID-19, meaning that the true estimate of the number of cases from 3/14 is likely around 270,000 cases (estimate range is 130K - 800K).
This also means that there were about 8.5 million cases in the US on May 1st (I happen to think mortality is closer to 0.5%, so that would be ~17 million cases instead), and that we've most likely crossed the 10 (or 20) million case mark by now.
How would we know? They are spread across the nation.