Does this happen while logged in to vCenter Server, and/or is it the same when you login to the hosts directly?
As you correctly assume, a LUN used as an RDM should not show up unless RDM filtering is disabled.
Are the LUNs presented the same way to all of the hosts, i.e. using the same LUN-ID? Maybe http://www.vdsyn.com/an-rdm-raw-device-mapping-presented-to-a-virtual-machine-still-shows-as-available-to-be-used-on-an-esx-host/ is also something worth looking at.
André