S40 Low Brake Fluid

Low brake fluid is never good. Have you checked the brake pads themselves? Worn pads will cause the fluid level to drop as the calipers extend further.

Yes, I considered worn pads. Given the symptoms, would worn pads alone typically cause such a significant fluid drop, or should I be more concerned about a potential leak within the hydraulic system itself?

Seen it plenty of times. Worn brakes will drop your fluid level quick. If it were me, I'd check those brakes and go from there.

Thanks for the advice. It turned out to be severely worn front brakes, as you suggested. The repair cost 480€ including new pads and rotors. I appreciate the pointer.