My bad, I posted a photo of a R3371A which has a similar RF deck to the R3265/R3271 but *not* the newer R3267/R3272. On the R3272, which has a much closer design to your device, the 10MHz reference is behind the 3rd converter, underneath the "3rd out" label.
Since removing the RF deck is a PITA you probably want to verify this first. Start at the synthesizer card. There should be 3 or 4 cards accessible after removing a panel on the right hand side (or what would be the right hand side were it sitting in the configuration it is designed to be used in). The synthesizer is the one with ~10 SMB connectors rather than 2 or 3 SMB connectors. Start at the SMB connector nearest to the back of the device and work towards the front, looking for 10MHz with an oscilloscope. IIRC the 10MHz reference should be the 2nd or 3rd you test. You can then trace the cable and will probably find that it does in fact disappear behind the RF deck.
Good luck, let us know how it goes!