In the Pyramid paperback editions, "Madame Maigret's Own Case," 1963 and 1971, Chapter 7, "Maigret's Sunday," page 110:
...And the next morning, because of the sun, which was again brilliant and springlike, Maigret sang in the bathtub.
In the Penguin paperback, "Madame Maigret's Friend," 1967, Chapter 7 (no title), page 109-110:
...And next morning, because of the sun which was again brilliant and springlike, Maigret had sung in his bath.
In the French, Tout Simenon 4, Chapter 7, Le dimanche de Maigret, page 280:
...Et, le matin, à cause du soleil, qui était à nouveau brillant et printanier, Maigret avait chanté dans son bain.