According to The sounds of Japanese (Vance 2008):
When /ɾ/ is the first phoneme in an utterance, the tip of the tongue is already resting lightly on the alveolar ridge, and /ɾ/ is produced by rapidly releasing this contact. Strictly speaking this utterance-initial allophone isn't a tap.
Can anyone elaborate on this utterance initial [ɾ] and its allophonic derivation, perhaps giving a reference for further reading? If it isn't a tap, then what is it? It sounds like it might be a voiced plosive, but it doesn't feel similar to either alveolar plosives [t, d]. It certainly sounds like there is an r-quality to it, but I wouldn't know how a plosive might acquire an r-quality.
Are you actually realizing, and furthermore aware of, an allophonic variant of [ɾ] (specifically the apparent utterance-initial allophone)?