Unmarked homework

Some schools now have a no marking policy where pupils correct their own work during class. This falls down when teachers do not check it is being done correctly.

Yes it should have definitely been marked. Some teachers are just very poor when it comes to marking (and I'm a teacher before I get pilled on.) Definitely don't be afraid to point this out to the head of faculty, it needs to be dealt with, and the teacher supported if necessary.

KS3 teacher here. Do investigate. Some reasons it's not marked might include:
- book not handed in (this should have resulted in consequence like d/t; has your son had any?)
- marked/discussed in lesson (pupil should have added notes & corrections)
- pupil absent on day(s) books were taken in to be marked.

Maybe the answers were checked in class?

Some of the benefit is in the doing.

But broadly speaking I expect homework to be looked at too.

I wouldn't 'complain', but I might 'mildly enquire', though first I would would check with your DC whether it is discussed in class at all.