Then this difference between the manga & anime is pretty significant. Also wonder why the anime has changed that.
I'm going to argue that it was to save time and to draw the season to its conclusion faster.
Most of the episodes of Sailor Moon S were filler to allow the manga to maintain a steady distance. However, as the season began to draw to a close, the anime unfolded its plot in a markedly different way than the manga, deviating even moreso than Classic and R. S is paced very oddly, with many of the major developments loaded on the back end of the episodes. Consequently, making the bad guys aware of Sailor Saturn's presence means there is less exposition when she appears later.
Also, the anime was trying to play up the whole awakening of Sailor Saturn would mean the death of everything, implying that she would be on the side of evil. While the idea of Saturn as a doom bringer was present on the manga, too, it was more of a neutral form of destruction. Basically in the manga, the characters and the audience knew that Sailor Saturn could destroy everything. In the anime, there was misdirection at play to get one to think that Sailor Saturn was working with the enemy directly.