Engine cooling system

About a week a go I was driving my Zafira (1.6 petro, l engine) back home when I noticed the car engine was juddering, possibly misfiring. I asked my brother what this could be and he advised me to check the water coolant levels. After opening the hood I notice the levels were empty. I immediately filled the coolant levels back up to the maximum mark and took the car for a drive. The car engine continued to judder/misfire whenever I pulled away or changed into a higher gear. After letting the engine cool down I took the car to my allotment (5 mins drive) and drove it back when I was done. When I got back I checked the levels again, while the engine was hot, and the coolant level was at its boiling at its brink. ( I think this was because I hadn't screwed the cap back on to its tightest point (tight but not tight enough.) I left the engine to cool for a good few hours before checking it again and when I did, to my surprise, it was empty again. There was water on the floor but I think this was from when the water was boiling out of the "not so screwed on cap." The next day I took the car to my mechanic and explained the issues I was having. I didn't want to fill the coolant level again in fear that the pressure would build as it did the night before and explode. I left the car with my mechanic who called me later in the day to tell me he had 3 of his mechanics look at the coolant system and none of them could see an issue. He stated the water is getting through the system and there is no blockage or fault and I should pick up the car with no cost. This stunned me because I remember the coolant tank being empty. When I questioned it he said there was definitely still water in the tank. When I collected the car I had to check myself and noticed that although the tank wasn't completely full, it was still between the maximum mark and the coolant tank cap (over filled.)

Eventually me and my brother drained the coolant and refilled it to the maximum level again with new coolant and some water. I also found the temperature gauge on the dashboard display. The car is still juddering/misfiring and is back at the mechanics today. Whats confusing me the most is that even tho we drained the coolant and filled it to the max point, revved the engine at 2,000 rpm's for a few seconds at a time, and gave the pipe a few squeezes everything seemed to be fine. We even checked the temperature gauge and noted when the fan activated, which at first was 108 degrees Celsius cooling it to a temperature of 94 (on a hot day, ) and now typically between 95-100 degrees. Last night when the engine was at a cool 35 degrees Celsius the tank was empty again with no sign of leakage on the engine or ground, and the engine is still running at around 95-100 degrees. Is it normal for the water to disappear like that when the engine still runs at an optimal temperature?

It maybe worth noting that the car is also in the garage to have a "minor" oil leak fixed in which the "exhaust manifold will have to come of to get to the oil cooler."