As your pads get used and worn down they get lower and the fluid gets lower as well. Being that brakes are 5 years old chances are it might be due for a pad swap. When you push the piston back into the caliper it will raise the brake fluid level. If you just top it off, there’s a chance you will spill out of your reservoir the next time you do a brake job if you’re not careful. Then it will be overfilled and you’ll have to remove fluid.