It was Bob and one other employee that interviewed me first, and I sent a single thank you email to both of them. The HR recruiter did not participate in either of these interviews, and for this reason, I am not sending a thank you to this recruiter. Is this the right move?
One of the members of the panel, I'll call Bob, is the would-be superior for the position, and gave me a tour after the interview. With this, I definitely want to send Bob a thank you email.
Would it be inappropriate to send only Bob a thank you email, and not the rest of the panel?
Short answer: Yes.
Given that you state that you want to thank for the tour, I think you answered yourself. As the other members didn't give you a tour, it wouldn't make sense to thank them for such, thus thanking Bob only seems logical.
Different case would be if you wanted to thank for the second interview, case where it would make more sense to email all the ones involved. However, I feel it would seem a bit out of place or odd to write (and receive) such email if you didn't have their contacts in the first place (in fact, the other members may be startled).
Furthermore, you mention in a comment that:
Their emails are listed on the company website. And it's only one panel member whose name I've forgotten. I remember everybody else's.
It is now clear that you had to manually
stalk search for their contacts, so sending them an email would surely make them wonder how or who gave you their contact information. Considering all these things I say it would be out of place to email the members of the panel that you didn't had their contacts before.
Another point to have in mind is that you already thank Bob and other person previously. It seems to be a common thing to send thank you letters, and the community seems to consider it Ok... however, sending multiple thank you letters, to basically as much people involved that you can (and sometimes more than once to some people) could be a bit too much, and worst case perceived as flattery.
I mention these things so you have them in mind when deciding if you send or not the thank you email(s). If you feel it could be perceived as flattery or out of place, I would avoid it. Perhaps what's best for you is to write to Bob only, thanking him for the tour.