Like Nick's situation, I met MrsTMRBB at work. Our relationship was concealed for a good 2 years before anyone found out - not that either of us were in a relationship to have to cover up - but we both agreed that due to our various positions within the company (no, not on the desk or over the photocopier,DS...) It would be "better" that way. No-one knew and that was PRECISELY because I didn't suddenly start making extra visits to her department and vice-versa. I still went to lunch with my brother and she still went to lunch with her friends.
Miss D, if you think it's becoming obvious, then it is - and not just to you.
Keep well out of it and cut these phone-calls out. You may think you are helping - with good intention. When it all comes out, you will be seen as a co-conspirator.
And that ain't gonna be good.