On all 208s that I know, the tank is in the Aft port locker.. If it is not there it is quite possible that the previous owner removed it and ran on premixed gas.
I assume that the motor is a 2 stroke. In order to run a 2 stroke without low oil alarms, the sensor wiring would have to be rigged to make the ECU think there was sufficient oil. The downside to removing the oil system is that the motor will run the same gas/oil mix ratio for all speeds. That might result in too much oil at low speeds and subsequent plug fouling and carbon buildup.
If you have a 2 stroke and no oil tank and have been running without pre-mixed oil, you will eventually blow the motor