Eseries low brake fluid

I had a similar issue with my old Renault Megane. Low brake fluid and spongy pedal. It turned out the brake pads were completely worn down. Hydraulic brakes use fluid to actuate the brake. As the pads wear, the caliper piston extends further, requiring more fluid to maintain pressure. This causes the reservoir level to drop.

Did you observe any other symptoms besides the low fluid level and pedal feel, such as noise or diminished braking performance, which would point directly to the worn brakes?

Yes, braking performance was noticeably reduced. Replacing the pads and performing a fluid replacement resolved it. However, it's best to get the entire brake system inspected. A workshop will be able to identify the root cause of your problem.

I had the brakes inspected and you were right. The front brakes were worn and caused the low level of fluid in the expansion tank. After replacing worn parts and a complete fluid replacement the final repair costs were 480€. Thanks for pointing me in the right direction.