Yeah, I dunno. You're right that there are remarkably few servants in SotL.

Does Alanna honestly pay that much attention to what ladies do? She seems to only notice them when they cross paths with her directly - like when she has to dance with them.

Also, I could plausibly buy Alanna not noticing if Delia and Roger were reasonably discreet. The palace is described as a rat warren; we also know there are hidden passages. Hell, Roger and Delia don't even have to be that discreet; they just have to be staying in parts of the castle not frequented by squires.