Holly--
Certainly worthwhile to check the fluid level (especially after all the effort you invested to find the tank!).
However, while I'm not a hydraulics expert I have worked on and repaired a lot of hydraulic systems in the past. Typically, low oil level would not result in the problem you describe. In your system, the pump pumps oil through lines into the hydraulic cylinders to extend them. If you have low oil level, and the pump runs out of oil and begins ingesting air before the cylinders are fully extended, they would simply quit extending.
If your cylinders are all extending properly--but later one or more "sinks" that would indicate that there is adequate oil to fully extend the cylinders, but later some oil "leaked out" of one or more cylinders. The leak usually wouldn't be an external one--it would usually be an internal bypass within that cylinder or (more likely) an oil leak back through the valve that closes to "lock" the oil within the cylinder after it is pumped in and hold the cylinder extended. Either fault would allow the cylinders to fully extend normally, but then let one or more sink over time.
Mike