REQUEST EXPERT HELP if you are not familiar with the tagger, especially
before removing anything that looks like a test setup..

To remove radioactive sources from the tagger,  go under the
tagger-magnet and look up into the gap between the scintillators and the
Al sheet that covers the vacuum window. There rails carry a slider which
can have a source taped on to it. The slider can be anywhere along the
focal plane, and could actually be hidden from view (at the high photon
energy end).
Also check outside the focal plane and remove anything else that you find
there, e.g. steel plates, plastic absorbers, soldering irons....