It doesn't matter whether you love your partner or not, after living with someone for 25years, you can surely understand their facial expressions. You might not understand what's going on in the person's mind or what they feel but it will be clearly evident on the face so I think Vanraj noticing that body language is absolutely normal.
Someone doesn't have to care or love to understand the body language. You can notice even strangers feeling nervous, happy, sad etc..then Vanraj has lived in the same house with Anu for this long, so him figuring out that something is fishy is very much possible. Also, he knows very well how Rakhi is so he was already in that doubt state of mind and Anu's silence made it clear.
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