I had anterior placenta with my first, and he was also a pretty quiet baby. I did worry sometimes because I'd barely feel him, and when I did it just felt like little motions.
It wasn't until I was PG with #2, who was very active and did NOT have an anterior placenta, that I realized just how much I was missing. Honestly, after my pregnancy with #1, I *do* almost understand how those "I didn't know I was pregnant!" stories can happen. If I'd been pregnant before and was expecting baby movement like #2, I would have written off most of #1's motion as gas bubbles.
However, while he wasn't very active, his heart tones were always very reassuring, which helped. I was also seeing midwives, who would palpate him and comment that even if I couldn't feel all his movements, he was moving during their palpations as much as other babies.