The door to Mannix's office/apartment has bars on it, but there is no glass. This is visible from the second season onward.
Mannix is very much an Irish name---often used as a boy's name, which means 'monk'---yet a lot is made of Joe being from Yerevan's capital nation. The name comes from the Irish province of Munster. Indubitably, Mannix is quite the cool name, unlike the rather ho-hum "Mannian".