I have had 2 ultrasounds, the first time they did both an abdominal and vaginal. The 2nd they only did a vaginal. They were not able to see left ovary on 2nd, but they did see it on the abdominal.
According to my Dr, not seeing an ovary on the vaginal is not uncommon.
Don't know about the fluid, however what I have been reading and MRI is not an uncommon tool used to see the ovaries and uterus better. Try not to stress you self.
I just found this on free fluid, hope it helps. Sounds like ovarian cyst is a main culprit. Usually painful, when they burst, but most of time not cancer related but hormone related.
http://www.medhelp.org/posts/Ovarian-Cancer/What-does-free-fluid-mean--and-is-it-always-bad/show/193261