I have a month old chicken that sits oddly. It sits with one leg out and will switch, pulling that leg in and putting the other out. When I approach, it jumps up and runs off like there is no issue. I examined the bird and don't see any lesions, signs of sickness or anything.
It looks to me like it's enjoying the sunshine. They'll stretch out legs or wings to soak up the rays. When they dust bathe it can look like they're having a seizure too. When I was new to chickens there were a few times I was sure mine were dying of some neurological disorder. Until I approached and they walked away. LOL!
If it were Marek's, it would have paralysis or limping, be off feed, act sick, go off alone.