I don't know if this will help any but ...
I was mowing with a finish mower rear deck and I went through a swale, a very shallow ditch. I thought I had enough clearance but I didn't and I grounded the blades which also lifted the deck against the three point hitch. There was a high pitched squeal, the mower blades stopped, the drive belt came off the deck and the back pressure on the PTO (or maybe the three point) blew hydraulic fluid up and out of the dipstick hole (obviously dislodging the dipstick too) which is aft and above the reverse HSD pedal.
I was finished anyway so I put the belt back on and parked the tractor after snugging up and rechecking the three point. I worked on the three point where I park the tractor, the tools are nearby. I tightened it back up and drove it around to make sure it was tight and didn't wobble or wiggle. Then I backed in to park in the normal spot again
This thing was laying under the tractor with the side that has the concentric rings (see pic) up and it was full of hydraulic fluid. I noticed it immediately when I climbed down, it's big enough to be real obvious. It's also big enough that I'm certain it wasn't there as I backed in because it would have been impossible to miss seeing it.
There's no chance at all that this is off the mower deck. 90wt gear grease doesn't look or smell like hydraulic fluid and besides plain old grease that's the only lubrication on the deck. There's no where for this to have come off the deck. It's pretty simple; a drive gearbox, a drive pully, idlers, spindles and blades ...
Something, either the PTO being forcibly halted or lifting force against the three point, generated enough pressure to blow a little fluid out of the dipstick hole and very likely that is what blew this cap off too. Then the cap had to have ridden around on the frame or hung up somewhere only to fall off just as I parked the tractor.