In this context, "off to bed" is ambiguous. The default would be on her way to her bed and, presumably, to sleep, but without details we don't know for sure.
It really doesn't matter though. The important detail is that the doorbell rang late in the evening, at bedtime, when most people wouldn't be expecting any callers. Everything else is superfluous, at least in the context of this sentence.
Another example, taken from the movie Home Alone:
The whole family was off to Paris for Christmas vacation when their mother realized she had left their youngest child Kevin back at home, alone.
If you've seen the movie, you'll know that the mother remembered her missing son when she was on the airplane -- but this one short description doesn't tell us any detail. The family could be anywhere on the trip. If it is important to know exactly what they were doing at the time, then the writer will add more detail in another sentence.