From the looks of it, that property is not documented, and I think I can see why. We only populate that when the TANK_CURRENT_PC env var is not set. The env var is usually set if you are launching Software via the tk-multi-launcapp:
TANK_CURRENT_PC
tk-multi-launcapp
https://github.com/shotgunsoftware/tk-core/blob/v0.18.172/python/tank/\_\_init\_\_.py#L35-L47
Maybe this would help: