I see it all the time and it happened to me this morning, on roundabouts - they can't be arsed if turning off, even though it helps people get moving,
On rural roads where it is quieter and a car is turning say right, no oncoming traffic, just me behind, car just swings right without notice.
The Rules
Mirror - Signal - Manoeuvre
The Reality
Manoeuvre, mirror, horn
If there is no-one else around, only you behind, why would they need to signal. Only if you where in a car and overtaking as you approached the road on the right, (which you shouldn't be doing anyway), would they need to signal!
If you where riding a bike, you must being doing some speed and/or doing something really stupid for a solitary car driver, ahead of you, with (as you state) nothing else around, again why would they need to signal. How would it inconvenience you?
If you are going to quote the M-S-M routine, at least find out how it should be applied properly.
You check the mirrors to see if you need to give a signal before starting or proceeding with a manoeuvre.
In the instance you quoted, I can see no need for the driver to signal, given the word picture you've described.
Where the brake lights illuminated? If the answer is yes, then they would have given a signal. As they where approaching a road on the right, with only you behind, the brake lights came on, slowing the car, you should be anticipating that they maybe turning. Could it also be that the offside rear indicator bulb had failed?
Something else to think on. Is your road craft so perfect, all the time, that you never do something that is not how others would do it?
Somehow, I doubt it. I know mine isn't. I make mistakes, we're all human. AND nobody would have been hurt.