It might be a crazy chance but it is possible your brine shrimp did have eggs, I'm a fairly new SW tank’er in comparison to some to the brilliance that resides here. However, Artemia a.k.a. sea monkeys are very hardy and if you have ever had a sea monkey set up as a kid or adult you see to condition that they come in, usually a liquid package of the eggs or larva, light sealed and on a store shelf for a year or so. The crustacean, Artemia, CAN survive a freeze and are fully anaerobic( no O2 needed), thinking back to my zoology class in college a few years ago, I believe that this particular organism ( in egg stage) is a facultative anaerobe, meaning they can use O2
if present. The eggs are not in of “normal” egg requirements and can survive extreme conditions. Where the sea monkey are found in the wild, the waters usually freeze so the species must be able to survive harsher conditions such as your freezer.
But who knows it may be something completely different…
Here is a pic of a Artemia: