Soma v

Hello all,
I own a 2010 w204 c250 cdi in india and have done about 146000 km . Around 6k km ago I had a problem with low coolant level and water pump , thermostat , fuel rail was changed . There was no problem until recently when I found low coolant warning again , 500 ml of water was used to refuel the reavoir upto the optimal level .on further driving 1000km later same problem occured . I examined the coolant system , no visible leak was seen . There was no oil mixture seen in coolant tank nor was a thick filament seen on the reservoir cap . On inspection of the dipstick there was no signs of coolant mix . There has been no increase in the running temp of the car nor white smoke from the exhaust . I've will post picture of finding on inspection of the car . I humbly request the memebers of this forum to provide their valuable input as to what could be the culprit and the subsequent action to be taken . Thank you in advance .

The cap of the coolant reservoir showed no signs of oil mixing


Coolant reservoir tank showed no sign of oil .


The dip stick showing the oil ( it's on the lower side , why though ? ) No signs of any deposits on the dip stick .

This tank (power steering fluid ) showed a leak , is this something to worry about ?

Picture of the engine bay , shows collection of oil in the spaces on the engine block which was present 6k km earlier . What could be the cause of this leak ?