[bobo5195]

Transpiration - water evapourating through the tube. Loop is closed so atomspheric pressure and the partial vaccum caused by there being less water in the loop causes the tube to be squished. undo the fill cap and see if it goes away i bet your tubes will expand and you will see there is not enough water in your loop.

Unless there are little goblins that nick all your water and squeeze your tubes which is of course a more fun explanation.