What does Acts 24:17 mean?

Paul is addressing the charges brought against him by the Sanhedrin to Governor Felix. The Sanhedrin's lawyer has accused him of stirring up riots, spreading a cult, and defiling the temple (Acts 24:5–6). Paul has already addressed the first charge by pointing out he'd only been in Jerusalem a few days and they have no proof or witnesses. He's disproved the second by explaining Christianity is not a cult but a different way of interpreting Jewish Scriptures (Acts 24:11–15). Now he talks about the temple.

Twelve days prior, Paul had accompanied several men, including Gentiles, who wished to bring offerings from their churches in modern-day Turkey and Greece for the church in Jerusalem (Acts 20:4; 21:18; Romans 15:25–26). While there, James and the elders asked Paul about a rumor that he taught Jews they didn't have to follow the Mosaic law. Paul denied the charges, and the elders asked him to help four men complete a Jewish ceremony as a show of good faith. Paul agreed (Acts 21:20–26).

While Paul was at the temple fulfilling the requirements for the four men's vows, Jews from the same territory as one of Paul's traveling companions saw him there and assumed he had brought his Gentile friend with him. They incited the crowd to attack Paul and the Roman army tribune had to rescue him by arresting him (Acts 21:27–36). Paul points out that these men aren't even present to give their testimony, inferring that the representative members of the Sanhedrin didn't witness the event (Acts 24:18–20).

We're not entirely sure how long it had been since Paul had been in Jerusalem. He visited after his second missionary journey before spending "some time" in Syrian Antioch. He then started his third trip by revisiting the churches in central modern-day Turkey (Acts 18:22–23). From there, he spent two years in Ephesus (Acts 19:10) and several months traveling around the Aegean Sea.