I take your point, Mad@urage, but as I alluded to in my reply to Mr Angel, the OP implies that the drivers had be alert and brake hard, rather than just slowing up to wait behind a cyclist as you suggest.
How do I know the cyclist didn't do a life saver? Because the OP couldn't see one. A glance out of the corner of your eye doesn't cut the mustard, I'm afraid. Moving from secondary to primary might be a handlebar-width sometimes, but mostly it's rather a lot more than that. In a narrow 3m lane the movement is about 1m, assuming a 60cm secondary and 1.6m primary position. In a wider lane it'll be more than that, and most lanes are wider than 3m. Even if you just move out a handlebar's width, it's still essential to do a lifesaver, and to look back, and to signal.
I'll bet Angelfishsolo would like to tell you why all of those are important, since he knows his stuff.
I'm certainly not saying that all cyclists are as aware of what is behind them as they should be, but shoulder-checks are not always what is wanted IME.
Cyclecraft covers basic shoulder-checks at level1. They are basic and need to be mastered;
they are not the be-all and end-all of checking backwards though! Cyclecraft acknowledges this, because "Many drivers seem to think that because a cyclist has looked behind and seen them, they may immediately overtake ... There are two possible responses to this:
If conditions permit a longer look back, look the driver full in the face and establish eye contact. This will often cause the driver to be more careful.
Otherwise, it is useful to acquire the knack of looking behind discreetly, so that you can obtain information without making it obvious that you have seen a following driver. To do this, move your eyes to the extreme but your head as little as possible.
Rearward observation is usefully complemented by listening, especially in quieter traffic conditions. Learn to differentiate between normal sounds and those that warrant your attention"
Cyclecraft disagrees with the statement that looking out of the corner of you eye doesn't cut the mustard - I agree with Mr Franklin, sometimes that is exactly the right thing to do. It also reinforces the point that you have to practice listening for the important information (again covered at level1). That the OP didn't see a shoulder-check does not mean a lifesaver look was not done. In a filter-lane I'd possibly avoid looking over my shoulder; I'd likely have done that well before, meaning I would know there are cars there. I would not invite an overtake by letting them know was looking again.
Moving secondary to primary through a lane-filter sounds very like riding along the lane through which the traffic is moving. From 1 metre from the lane edge to centre of a 3metre lane is a 0.5 metre movement. following that into the centre of a filter lane is to remain in the centre of the traffic flow. Neither should require a signal, just good control of the traffic around you (which can mean forcing them to slow down). As it says in cyclecraft "Follow the traffic, not the edge of the road".