Sorry to complicate matters here, but there seems to be more going on here than just dynamic modality but I don't think it is epistemic or deontic, either.
To my mind using "could" in this way implies that not only was she:
1. able to see the bus pulling away, but
2. she did in fact see the bus pulling away.
I would say there is less going on here than you think, not more.
She could see the bus pulling away
is to all intents and purposes the same as
She saw the bus pulling away.
Compare this to,
3. "A true gentleman is a man who can play the bagpipes but doesn't."
A few comentators have said that there is a fourth type of modality to do with facts called existential modality.
If we say that the statement in (1) is dynamic modality, I think (2) might be an example of existential modality. Confused
Consider things less deeply. The type of modality you are referring to in (3) is existential modality (a less recondite example is
He can be very helpful.). Treat it as a sub-class of dynamic modality.
Let me repeat Andrew, the advice I gave in one of the threads above. Put any sentence you are unsure of into the third person and ask
Who is calling the shots?. (With regard to the modal verb of course).
If the modal verb is a property of the subject of the sentence and not the speaker or the person addressed, then it is dynamic modality.
Examples:
They may help you. Epsitemic modality dealing with possibilty. It is
the speaker who is stating their helping is possible, not definite.
They ought to help you. Deontic modality.
The speaker is imposing the obligation on them to help you.
Can I come in?Deontic modality, asking for permission from
the person addressed.
He can help you.Dynamic modality.
The subject is the one with the ability to help.
Huddleston, in "A Students Introduction to English Grammar" says that "dynamic interpretations are somewhat perirpheral to the concept of modality" so perhaps we can say that dynamic modality is what modal auxiliaries manifest when they are not being modal.