Weird coolant problem

I haven't experienced this problem before. 
The low coolant alarm sounds when I go to start the engine, it's the constant beep.
When I go to check the coolant it's like there is pressure in the system or, thinking about it now, a vacuum, the tank then refills and normal service is resumed the car then drives perfectly without any issues. 
What is causing this?

If it is pressure, it could be a weak hose ballooning, lowering the expansion tank level.

Thanks David. 
1st time it happened was a cold start after sitting overnight, I was getting the constant beep so I went to check the tank and when I undid the cap I got a hiss like the car was hot and the overflow tank refilled. 
2nd time the car was warm and I got the same, cap off and the tank refills.
I have driven the car again today without any issues. 
This 1 has me stumped.
Could it be prt related?

Since the liquid coolant is uncompressable (not strictly true, but would require pressures far in excess of what could be contained by the coolant system of a car), for the contents of the coolant tank to disappear into the coolant system under pressue, either the coolant system is expanding in capacity (as per David's suggestion of a balooning hose), or there is a significant pocket of air/gas elsewhere in the system that is being compressed and replaced by coolant. Removing the cap then allows either the hose to contract, or the trapped gas to expand, forcing the coolant back into the tank.

I would probably start by replacing the tank cap, in case the vent is blocked causing the pressure to rise well above what is normal (which can cause even good condition hoses to baloon), then bleed the coolant system. If the issue persists after that, I would carry out a sniff test to try and rule out combustion gasses entering the coolant system.

Hi I would be inclined to do a complete drain down, save the coolant filter it then refill the system and bleed thoroughly. That should rectify the problem.
(I filter and reuse the coolant in my cars if it looks CLEAN!!)
Good luck. G.

Have you checked for a blockage where i suggested ? No one thinks of it on the opposite side of the engine but it becomes a more obvious candidate when you realise that it is right by the hotest point of the cooling system and if flow to the bypass gets blocked then the thermostat will never become hot enough to open until the engine has already grossly overheated.

Thanks everyone ☺️ 
lots to look at but I'm sure that the solution is in one of your suggestions. 
The previous owner wants to buy the car back and I want to make sure that it's good before she has it. 
I will update with my findings when it's sorted.

You are doing exactly the same as I did with my Mk1 F. I went the extra mile to make sure it was spot on so her faith in the car would be restored after the bad experience she had with incompetent mechanics making it worse

I replaced the coolant cap and everything seemed fine, even during the recent hot weather I had no issues with alarms or overheating on a few long runs. 
However.. the car has now developed a misfire on start up, really noisy lifters ( unit warm)..and the coolant problem is back!

 

Well I was hoping for a better outcome but..
I'm glad that it's happened before the lady got her car back. 

Its not a nice moment of realisation at all. Will you plan to get the head off next ?

Yes I will be.
hopefully I don't have any issues with liner heights.