A French case report that has received widespread media attention suggests (actually stated in pretty strong terms in the paper) that Covid-19 was already spreading in France in December 2019. (In contrast, according to the received account/wisdom from the WHO, the first recorded case of Covid-19 outside China was on Jan 13 in Thailand.)
The retrospective PCR test for Covid-19 they did on the -86C frozen sample had an amplification curve like this:
Fig. 2. Result of Charité protocol RT-PCR assay : patient's curve in yellow; positive and negative test samples in purple.
A Ct of 30 seems pretty marginal to me, but I'm not really an expert. What does the Charité protocol research say about such a Ct figure, in terms of false positives, i.e. what's the probability that a Ct of 30 (or more) is a false positive using the Charité protocol/test for Covid-19?
(The ref/link  in the paper alas doesn't send to a useful page in that regard.)