Trans oil gone?

Got a 2001 Cat 120 grader that's burning through transmission fluid. It drops to near the bottom of the dipstick about once a month, sometimes faster if we're running it hard. We can't find any leaks anywhere, so I'm stumped. The Cat dealer just told us it's normal and to monitor it, but that doesn't sit right with me. Has anyone dealt with this before? Any ideas what could cause it?

Oil doesn't just vanish into thin air—it's got to be going somewhere. Are the fluid levels holding steady elsewhere? The leak's either internal or external, got to be one or the other.

What's the serial number on your 120? That's odd that the dealer would say that's normal.

First, I need to correct myself. This is a 120M2, transmission serial number LSZ01107, arrangement number 396-8363. All the other fluids are staying at normal levels. Cat told us it can't be making it to the engine and getting burned, since that would also affect the aftertreatment system. We don't have any leaks that we can find. That's what makes this so frustrating. The operator's been adding fluid to it as well. We're taking samples every 6-9 months and they're coming back good. Nobody here has figured it out yet. I thought maybe there's a high-pressure pinhole leak somewhere that could be spraying fluid, something you might only notice when it's in gear and running down the road. But we still have no idea what's going on. The dealer's looked at it twice now and we're still stuck. Thanks for any input.

The grader serial number is C7R13722, model C-7.1.

Have you checked the oil levels in both tandems? Any oil showing up around the tandem breathers? Got any photos?

Have you checked the output shaft seal carefully?

I haven't checked the tandems yet, but I'll look at both of them as soon as the machine gets back here. Thanks for the tip.