Good that you found that - but now the issue is why? For me I'd need to get underneath and inspect all of the brake lines, especially around the calipers for signs of leakage etc.
Fluid will only go down if it's got somewhere to go. (Hopefully it's just taking up the slack as the pads wear - but you have to know).
Some places do a "free" brake check. Never used it but from the Halfords website:-
"Our free brake check covers all components in your car brake system. Our ATA-trained technicians look at brake pads, shoes, callipers, hoses, discs, and handbrake linkages.
If brake repairs are required, we'll let you know why, and we'll advise you of costs upfront."
Tell 'em to look but don't touch!