On a day that the Doctor happens to be away from the TARDIS, the Master wakes up with unexpected energy. He decides to rummage about in the supply cupboards, where he finds an ancient and disabled vortex manipulator, as well as a few of the Doctor's old tools. After some research and many calculations, he sets to work.
The TARDIS key hanging around his neck like a dog-tag clinks annoyingly against the devices. Finally, he yanks it off... and pauses a moment, turning the key between his fingers. How long will it be until he's well enough to survive far away from the Doctor's TARDIS? What's it going to take to trace where and when his own timeship is hidden and get back to it? If he leaves, could he ever come back? When he leaves, will he ever have somewhere to rest again? But today, the Master has had enough of resting. He uses the key as a bookmark, to keep from losing his place.
One day he'll disappear, leaving behind the electronics repair manual with his notations. If the Doctor wants to find him, it shouldn't be that difficult for someone of their intelligence to track him down.
He tries entering a few coordinates on the device, without activating the travel function yet, just to see if it can plot the route. Circuits ping and fizzle and an electric jolt throws him backwards. He eases the device off and scrambles upright again, cradling his shocked arm and cursing the inferior equipment.
Eventually, he makes a note of what went wrong.
Testing is important.