For any of those studies, look for the specificity of the test, which is basically the false positive rate. A serological test with 2.5% false positives is cons

For any of those studies, look for the specificity of the test, which is basically the false positive rate. A serological test with 2.5% false positives is considered a pretty good test. If the test used in Stockholm were similar, they could have zero cases and get that result.