Let me say it again. "The Scriptures" were the Old Testament books - the prophecies about Jesus. They searched to make sure the prophecies had been fulfilled.
Obviously they only had the OT. What they were verifying was the gospel. Paul spoke for 3 days in the verses before to those who were not as commendable, he didn't just say a little. And again, when they search the OT scriptures daily to see if the things Paul said were true, if they weren't in the scriptures they weren't.....