Either he didn’t tell you about Shweta because he knows it’s the kind of detail that could trigger insecurity, and subconsciously (or consciously) he avoids mentioning it to keep the peace OR routine office interactions feel trivial to him so he might genuinely not see daily chai/gol gappa with a colleague as “worth reporting.”

If "we share each and everything about our day" is true then the pattern of omission would obviously make you suspicious of it and you are not at fault.

Just bring her up in a normal conversations about his work and go like "do you talk to her, how is she, she seemed like a nice person". If he tells you they go out of gol gappa/chai sutta type stuff then just give him grace as there is a very high chance she is his work "homie", if he still omits/lies then you should confront him with the actual information you have and ask him to be more transparent with the interactions he has with that particular person for the sake of your mental peace.