If someone was ill, then they're back, asking them if they're feeling better is totally normal conversation.
The suspicion etc may not be, but the question is obviously fine.
Given you're suspicious about these people and the way you talk about them, they are most likely well aware that you're on a fishing trip feigning interest.
People like you are always more transparent than you realise.
Apologies if it wasn't clear, I am a different person to the one you replied to, and am not suspicious - I was commenting on the other person's suspicion and indeed said that it wasn't normal.
I'd say you're missing the hint that it is none of your business.