One of my LinkedIn contacts recently messaged me if I was looking for a job change. He is not a recruiter but a principal engineer in some firm. He appears to know some senior people at my current employment because in the same message, he mentioned some names and asked how they were doing and all. I am looking for a job change for sure, but quite hesitant to answer this direct question, since dont know the person personally.
If you are not comfortable with being straightforward and want to play diplomatic, you can answer something like:
"I'm not actively looking for a change right now, but I'm open to discussions".
Then, if the discussion starts, you can actually show interest. There's nothing wrong in that.