Citroen DS3 coolant loss with no leak, no overheat, no oil/coolant mix, no mayo cap

Hi,

I am looking at a car that I want to buy, but there is a problem with the car that the seller has been transparent about. It's a 2015 DS3 Automobiles DS3 Dark Light 1.2 110 bhp Turbo

- Petrol, Automatic, 76,700 miles
- Full Service History (6 Main Dealer/4 Independent)
- New Timing Belt at 54,000 miles/June 2024
- New Battery Sept 2025
- A few weeks ago, the engine coolant temperature warning appeared. When checked, the coolant reservoir was empty. The car did not overheat. Approximately half a litre of water was added, and the car was driven home without issue. The following day, the coolant reservoir was checked again and found to be empty, again taking around half a litre to refill. The car was then booked into a garage for inspection. A coolant pressure test indicated a leak; however, no visible leak could be found. There were also no signs of a head gasket issue (no oil/coolant mixing and no emulsification under the oil filler cap). Without further investigation, the garage was unable to confirm the cause. Since then, there has been no further coolant loss.

I'm on a tight budget and need a car for work. This car is really cheap compared to the same model on the market. At first, with my budget, I plan to buy a car that is around 100k miles and use it for 2 years, but when I see this car, it only has 78k miles, so I thought if there is no serious problem, I could fix it and keep it for longer. Thank you in advance for your help and advice!