wouldn't the zoo she came from have had an idea that she may have been pregnant... they don't just appear inside. she must have had access to the bull there before she came over
i'm guessing she was still with her mother in the main paddock (san diego run large groups of rhino together don't they) with either her father or another bull - and that she was removed before the (assumed) age at which she could fall pregnant.....
One of the Berlin Zoos(east, I think) also received a female Indian rhino some years ago from San Diego, which was similarly later discovered to be pregnant. pregnant. In that case it was a happier outcome, a female calf was successfuly produced and is now the zoo's 2nd breeding female....