There is water at the base of my upstairs bathtub right at the edge after a shower or bath, especially at the front end nearest the faucets. Could there be a leak from underneath? There is no evidence of leaks on the first floor ceiling.

Could be a drain leak, supply line leak or even a mixing valve leak. We need to start eliminating these one by one. Fill a bucket or 2 with water from another room. Pour the 2 buckets of water into the tub to test the drain line. What do you see?