Luke 1:22

‘And when he came out, he could not speak to them, and they perceived that he had seen a vision in the temple. And he continued making signs to them, and remained dumb.’

And when he did come out they waited expectantly for the priestly blessing (Numbers 6:24-26) which would normally follow the offering of the incense. But it soon became apparent to them that the blessing was not coming. They realised that Zacharias was unable to speak to them, and they gathered that he must have seen a vision in the Temple, for the only way in which he could communicate with them was with signs.