He was too well trained not to feel the intrusion, and the Dark Lord was doing nothing to mask it. And so he showed him what he wanted to see. The meeting with Dumbledore, or rather parts of it, modified in his mind to a different truth. Asking about the symbol, Dumbledore's lies and pleasant manipulations in return, his own doubts.
And then a new scene, created from several meetings over the last few months, filled with frustration and dislike. Him worrying about the Elder Wand falling into the wrong hands, and snapping that it was certainly safe with him, that 'Voldemort was only too welcome to try and take it'. A casual question about marks on the wand leading to a confused and equally annoyed no, of course not. The answer was the truth, only to another question. But Theodore had long since learnt that for a lie - spoken or shown - to work, you needed to be close to the truth.
His mind flipped to other memories. Of his daughter, though he only allowed Voldemort to see a glimpse of that before blocking him, knowing anything else would lead to suspicion. Instead the murderer, the weapon, terrified while trapped in his dungeon, him reading his mind and then leaving him alone, sobbing on the dungeon floor.