This type of behavior often peaks in adolescence and is more common in male rats. It typically decreases with neutering. There is a lot of information contained in those little drops of urine, such as information about identity, age, sexual maturity, social status, and stress levels. This serves both to mark the territory of the rat and to communicate other things to other rats. It is a bit of a nuisance but very normal and typical behavior.
A rat that is startled or nervous may urinate larger amounts, but that is a different response than the marking, in which small drops or smears are left as the rat moves about.
