I don't know specifically for the 712+, but some of the DS models indeed keep the fan spinning even during hibernation (e.g. the 210j). Some boxes might just need that if CPU (which is always running) needs continuous ventilation.
DS-411 (DSM 4.2-3202) w/ 2x WD10EFRX + 1x HD154UI
DS-106j (DSM 3.0-1357), PATA-to-SATA adapter, 2.5" HM250HI