some decrease in the reserve tank is normal...

however if you can visually see coolant traces/leak then definitely need to get it fixed, doesn't matter where it is.

The car cooling system is suppose to be a closed loop. Small leaks can lead to major failure in future.

Most of the time small leaks will quickly evaporate due to the hot engine... so that make it difficult to spot. But coolant traces is a tell tale sign something is wrong