Omnipaque used to be the norm for adjusting the Swedish AGB which is now the Realize band... and yes there is a chance for your band to lose Fluid over time without your having a leak at all. Because omnipaque is a thicker, more viscous substance, it is often used in those cases to keep the band inflated without it continually or eventually losing it's fluid.
I'm not suprised that you haven't had this substance in your band, (as far as i remember as it may have changed in the past few years since I have been out of the business) as it is not approved for long term use in a band and will actually void the manufacturers warranty if not flushed out directly after leak detection under fluoroscopy.
That said, it is not uncommon for fluids such as omnipaque to be used to adjust the band, but it is uncommon in the USA.
And Cheri, adding saline to the band which has been previously adjusted with omnipaque will only make the solution a tiny bit less viscous. If you prefer it stay in there, just tell them.