The Golden Orchid
joined Jan 19, 2017
she put her hand on Aizawa's shoulder, and she definitely stopped Michi from asking more questions that might lead to Kasumi realizing she was not herself for a moment there with the possession
Putting her hand on Aizawa's shoulder seems perfectly in-line with not realizing anything. She walked in after Aizawa stopped possessing Kasumi, and Aizawa's wearing the school uniform, so she would have just looked like a random student who was there with Kasumi and Michi for no good reason. Sayaka noticeably also immediately blames Michi for the strange position she and Kasumi are in, not Aizawa.
And she therefore probably wasn't stopping Michi from saying anything that would let Kasumi know she'd been possessed, but rather anything that would let Kasumi remember how she'd been snuggling with Michi. Simple jealousy.
Of course she could just be an absurdly good actor, but I don't think there's anything that contradicts what's being shown of her being truly oblivious. Michi had the ability to see ghosts since childhood, so Sayaka's likely the same and that's why she thinks everyone can see them.