Severus's mind was well shielded as he intended to test how well Theo had been practising in the time he had been at Malfoy Manor.
He felt Theo enter his mind and probe at his shields. The other man was good, certainly better than he had been when they begun and it was clear he had been practicing. He was still far too clumsy but that was a common problem. Few ever reached the level where they could insinuate another's mind unfelt. Few ever needed to reach that level.
He let Theo test himself for a little longer then cast a light hex and turned the spell back.