Noticing humans being sneaky was a survival skill for Bastion early in life and they've held onto the habit of paying attention as best they can to who's in their presence (to the point of unhealthy neuroticism), so I think they would have seen Cassandra sneaking around at least once, but if she's a skilled sneaker she could have successfully snuck past them at least once as well. They're hypervigilant but their senses aren't dramatically more acute than a human's beyond having access to zoom and thermal imaging. If they noticed her, they wouldn't make it obvious by waving or staring or flinching, but she might notice them keeping her in view of their main optic or subtly moving out of her line of sight.