Ducks can be active or asleep at any time of day or night. They don't sleep through the night like chickens. They will party anytime! So, your ducks are just doing what normal ducks do
mine run amok on my porch after the sun goes down chomping delicious moths. Actually if you want to save a bit on your feed bill I recommend getting a very bright solar garden light and setting it up inside their pen for a nightly smorgasbord.
I love it when I get home late and my chickens have put themselves to bed- and are quite perturbed when I check on them and wake them up- but my ducks come running out of who knows where with loud greeting quacks like they were waiting for me to start the party