I've heard aircraft using the call sign "Tango" for part 135 ops.
From Wikipedia:
Air taxi operators in the United States sometimes do not have a registered call sign, in which case the prefix
T is used, followed by the aircraft registration number (e.g.
Tango- November-Niner-Seven-Eight-Charlie-Papa).
Unfortunately, that does not explain what you saw, with the "T" being painted on the side.
T9 is the registration for Bosnia/Herzegovina. Could it have been T9-2NC?
List of aircraft registration prefixes - Wikipedia, the free encyclopedia