You check the reservoir, the fluid is low, but you cannot find a drip anywhere. In most cases this has a simple, harmless explanation. Here is why brake fluid is low with no visible leak.
Worn pads are the usual reason
As brake pads wear, the caliper pistons extend further to reach the rotor, and that space fills with fluid drawn from the reservoir - so the level drops naturally as the pads wear; see low reservoir. Fresh pads bring the level back up when the pistons retract.
Do not just top it off
Because worn-pad-low fluid is being held in the calipers, topping it to MAX means it can overflow when you push the pistons back for new pads; see brake service. Check the pad thickness first - if they are worn, plan a pad job rather than adding fluid.
Rule out a slow leak
If the pads are fine but the fluid keeps dropping, there is a slow internal or hidden leak - a weeping caliper, a line, or the master cylinder bypassing; see fluid leak signs. A falling level with good pads is not normal.
What to do
Match the low level to pad wear - if the pads are worn, replace them and the level self-corrects. If not, hunt the leak. Fit quality pads and hardware.