Any time my ducks make sounds that are new/different, I keep an extra eye on them. It may not be a problem, but understanding your own flock and learning their “language” can give you insight in case of later issues.
First time I heard a drake whistle, I thought it had to be coming from somewhere else, it sounded so odd.!
I commend you on being observant and then being concerned like @WVduckchick said anytime we see or hear something off it’s always best to keep an eye on this. You got right on it.