First, I highly disagree that marshals and generals don't need to know how many siege engines the army has. The success or failure of entire campaigns can hinge on how many siege engines an army is carrying. What a knight considers "enough" may be nowhere near what a marshal or general considers "enough". Yet the marshal/general is the one responsible for making sure that the siege succeeds.
Second, siege engines are not small things. Siege towers are scores of feet tall. Siege ladders, as well. Catapults and trebuchets? Not small. If you can see a unit well enough to count the number of men they have with them, then you damn well can count their siege engines.
Banners, too. And probably caravans as well. These are rather large freight wagons. Kid of hard to hide.
Now when it comes to thing like scouts and healers, I tend to agree that they probably should not be listed. Possibly not carts, either. After all, every traveling troop of armed men will probably have some form of cart or pack animals to carry equipment and supplies. And when you get up to the scale of armies, you have the proverbial camp followers that trail along with the army.
This also means that these things would be visible when you scout enemy troops, as well.