Audi A3 8v 1.6 TDI weird cooling issue

[mbmbss]

Hey, I came across a weird issue that's left me and pretty much everyone i've talked to stumped

Just some preface: An Audi A3 8v 2013 1.6 tdi about 84000 miles, never had the cambelt changed or the water pump changed + I bought the car about a month ago

I've been driving the car for about a month straight taking it on the motorway a few times and all had been good that far

It was on a longer motorway drive where I had got an alert on the car saying that the car's coolant was hot and too pull over (I can't remember this alert as much but it was to do with something being hot) the temperature was showing about 120 degrees celcius, I pulled over and my dad decided to open the coolant tank bottle. There was steam at the top of the bottle but still coolant in there, and as soon as it opened the temperature dropped within a minute to 90 degrees celcius. 

We continued driving as we were on the middle of the motorway and couldn't make sense of what happened, and then after a few miles the car said the coolant was low, when we opened the coolant resovoir it was completely empty, we had to fill it with water (that's all we had), we picked up some coolant from the service station and continued the drive. 

Another few miles down and the coolant was completely empty again, so we had put the entire bottle of coolant in and kept driving. After that, the car's been perfect, it drove all the way there perfectly fine and on the way back (1:20 drive) able to stay at about 70-80 mph perfectly fine. We checked if there was coolant on the floor after we pulled away and there was nothing.

Went to the mechanics and he couldn't notice anything wrong, even putting the car up, (but he did only take the undertray off), he took out the coolant and filled it up with his own and told me to keep an eye on it. 

Any idea's what this could be? Or has anyone faced something similar?

[Steve Q]

Has it done it since. The system definitely needs checking thoroughly. It could be an incorrectly sealed expansion bottle cap. That'd be your cheapest option. Is there any white smoke out of the exhaust or what looks like mayonnaise around the oil cap?