The EM-50 Listening Device is a bug used by the NSA and, in particular, Casey to monitor Chuck in the Buy More.
It features most prominently in Template:113, when it is revealed as Casey's bug of choice. It is similar to, but less advanced than, the GLG-20 Listening Device.
Nerd Notes
The EM-50 was the name of the Urban Assault Vehicle from the 1980 movie Stripes.