2002 MP5 boiling coolant reservoir. WTF?

Pulled up this evening after a short ride to discover my coolant resevoir boiling, no indication of engine overheat on the instrument panel, Recently purchased this vehicle 3 weeks ago, and have put maybe 600 km on it have not had an issue until now. any ideas?

UPDATE Refilled coolant and reservoir tank following morning, burped the system as rad was extremely low, drove maybe 10km and overheat issue again. allowed to cool for an hour, drove another 10 min rising temp gauge, pulled over let it cool, and started vehicle back up, ran it for 20 min checking main fan functionality also discovered lower rad hose extremely cool even when temp gauge reading half way WTF? bad thermostat?