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Brake fluid below the minimum in the reservoir. There are only two explanations and they need separating immediately: the pads have worn, or the system is leaking.

Wear is the benign one. As pads wear the caliper pistons extend, and the fluid filling behind them comes from the reservoir — so a level falling slowly in step with pad wear is normal and is corrected by fitting pads, not by topping up. A leak is the other, and it is a safety fault. Check the pads before adding anything: topping up a system whose pads are worn means the reservoir overflows when the pistons are pushed back. A level that falls with pads in good condition means a leak, and the vehicle should not be driven until it is found.