Well, here is a real long shot.... but the color of the plastic reminded me that I have an old Toyota Land Cruiser that has that same color of yellowish weathered nylon-looking plastic held in place just inside of the filler cap for the power steering fluid reservoir. It is just an inserted bit of plastic used to hold a filter pad up against the vent hole in the cap. I suppose it could drop down into the reservoir and travel from there.
Anyway, I'd look at the inside surface of any of the filler caps for the hydraulic system.
rScotty