Time Machines In Transit

Why are time machines invisible in transit? Are they always?

(i) HG Wells' Time Traveller tells his dinner guests that his Time Machine moves too fast to be seen but it doesn't move anywhere. It remains stationary on the Earth's surface in a state of time dilation so it should resemble a statue. The "Time Traveller" does not travel anywhere or when. In The Space Machine, his sequel to The Time Machine and The War Of The Worlds, Christopher Priest explains that the Time Machine in transit is attenuated.

(ii) Doctor Who's TARDIS disappears into and re-emerges from the "time vortex" where, according to the villainous Master, E = MCcubed.

(iii) Poul Anderson's Time Patrol timecycles do not exist between departure and arrival. They disappear from one set of spatiotemporal coordinates and (re)appear at another.

When a timecycle appears/arrives, it might have disappeared/departed from:

same place, earlier time;

same place, later time;

different place, earlier time;

different place, later time;

same place, prevented future;

different place, prevented future -

- but it cannot arrive from a prevented past.