Starlight
Donald Noble had never been good at spying and it was a pleasant surprise when none of the Time Lords he was observing - watching, spying on, whatever you want to say - noticed him.
The door of the TARDIS was wide open and all Don could see was a bright light surrounding their figures, which the Doctor later explained was in fact the light of an expanding star. Starlight.
The Doctor was leaning on the right hand side of the TARDIS, watching the sight in front of her without turning to look at the man next to her. The Master was looking at the sight as well but slightly turned his head to look at the Doctor, causing Don to smirk. He was getting some good blackmail material today!
The Master suddenly turned to look at the Doctor fully and whispered something Don couldn't hear; he could only see his mouth moving. He didn't see the Doctor's reaction or reply; what he did notice was how she took the Master's hand, her head still looking at the star.
None of them said anything again and Don left as silently as he came.