Whether she's seen another duck is irrelevant, the instincts of wanting a duck reply are still there. I keep some buffs with my Muscovy flock, they communicate with each other the most, while they enjoy their scovie companions, the voicing is completely different and they look for ducks of their own kind(Muscovy are NOT mallard derived and basically geese) so it was quite humorous when they came across the calls i keep, and conversation came about because they speak the same language.
As well ducks are chatty and loud, this is the way they get themselves heard, my buffs and calls are always talking, always saying something and the voices vary, these ducks especially the buffs are in flocks so they have many birds of their own kind.
Frankly, singular ducks will always be more noisy regardless of whether they have fowl with them, again the instinct to look and get a reply from someone of their own language is always there. If the duck acts normal otherwise, and is eating/drinking/bathing i don't think it's anymore than that, she is likely trying to gain a proper duck response from you.