Hi Sassco,
You will find that your meal of Salad, Sausage & Egg contains only foods from the Vegetable and Protein food groups therefore they will not alter your blood glucose level.
Your Blood Glucose will only ever be affected by Carbohydrate foods, so Bread, Rice, Potato, Sugar etc will all have a raising effect on your BG.
Proteins, Fats and Vegetables/salad will have no effect (Neither will Nuts)
The drop in BG is because you had nothing in your meal to raise/maintain your BG level, but with the list of treatments your taking you will certainly have had something there that was at the time having a lowering effect on your BG level.
Hope that helps make some sense of it for you!