That's what I'm thinking. It would be the hose that comes straight forward under the seat. Reach back in there with a small screwdriver or something. If you feel threads back in there,the plug isn't in there. It's a few inches back there,so get in there deep. If it's not there,get a 3/8ths (or maybe it's a 1/4,I forget) plug that uses an allen wrench. Stick it on the end of the wrench and screw it in to the threads.
Don't know if that's gonna be it or not,don't know how it'll act without that plug in there,but that's what I'd check. A tip though,don't ever start the tractor with that plug in there without running a jumper hose back in to the inlet if you take the loader off. There's nowhere for that oil to go with the plug in back there except out the front where your loader hose hooks.