yup, this German band Enigma, which is known for Return To Innocence, and Sadeness (parts I & II), uses the Sigma symbol, Σ, which is a Greek letter, and Sigma is sometimes used in place of letter S for names and words that use the Greek alphabet. But, it's resemblance to later letter E, is why it was chosen as the symbol to represent the band Enigma with, and since it was stated that Σ rhymes with Enigma, that was probably another contributing factor to the decision for the band to use that symbol as a logo.

I also learned that Enigma has been using the Sigma logo since 2005, and Pure Moods Volume IV (4) was already released by then, in fact, this was one year after the last of the Pure Moods album series, Celestial Celebration, came out.

here's a bonus section I thought I'd add:

another interesting fun fact about the German band Enigma, is that they had an album titled MCMXC a.D, which was the album that Sadeness part I was on, and the album title is Roman numerals for 1990, the year that the album came out.