Brake fluid disappearance

2014 Ford mustang GT. 6 speed manual. Does anybody know what can cause brake fluid to just disappear. I've been searching for a leak but haven't really found any signs for what could really cause this issue. I've changed out my brake and clutch pedal assembly around 2-3 years ago and that came with a brand new master cylinder, I've also installed a new twin disc organic clutch 3 years ago but just recently around Thanksgiving last year I started losing pressure in my clutch pedal. I checked and saw that my brake fluid was low so I topped it off again but it's like I have to do this every 3-4 weeks. I can't find any obvious signs of leaking anywhere so can somebody maybe give me a suggestion on where to look? Or maybe it's a failing part?