J.T, I'm going to go out on a limb here and take a wild guess. Since Hibisicus has few pests and none of which would cause the damage you are referring to, I'm going to point my finger at a highly unlikely culprit. The rabbit. Rabbits love hibiscus and if the stem is cleanly cut off like a knife did it and the cut is about 6 inches off the ground - odds are its a rabbit. If you have a screened porch then forget what I wrote. As I said, it is a wild freak thing but I don't know any other pest that would do this type damage to a hibiscus.