Acts 4:13

‘Now when they beheld the boldness of Peter and John, and had perceived that they were unlearned and ignorant men, they marvelled; and they took knowledge of them, that they had been with Jesus.'

We only have recorded the words of Peter, but it is clear from these words that John had also spoken (the boldness -- of John'). And the Sanhedrin were impressed. They were used to men cringing before them, not speaking out boldly. And they were not used to having Scripture quoted at them. The fact that these were ‘unlearned and ignorant men', that is, not officially taught in official methods of Scriptural interpretation and not cognisant of the Law as officially taught, meant that they could only be officially admonished. However, once they learned that they had been with Jesus it put them on the spot. They had rejected Jesus as a heretic and a blasphemer. But these men were still proclaiming Him and even claiming the power of His Name. Now therefore it was apparent that they were Jesus men, and that they were representing themselves as carrying on His work.

Most, if not all, of them had probably never previously noticed the disciples. Their attention had been on Jesus. Thus it is not surprising that they had not recognised in these bold men the previous rather timid (in the presence of leading Scribes and Sadducees) followers of Jesus.