A simple "thank you" unethical? OMG how did we get to this place?!
It would be perfectly fine and not at all weird.
As long as you are simply thanking them for acknowledging your work and not making a big deal about it, it's simply a normal, decent thing a person can say to another person. We are after all still human beings.
If you add some additional chit-chat or mention something related to research they might find of interest, or "while I've got you here, I'd like to as a question" it may help to make your message feel more normal to you and to them, if you feel it's a concern.
Academicians should not live in balkanized ivory towers, they should be able to feel a sense of community and camaraderie1, and how better to do so than to treat each other in a warm and social way.
How can a simple "thank you" be anything but good?
Beyond that, you may make a friend, or find further direct discussion useful, or even collaborate some day.
Consider it an ice-breaker; go for it!
1though it may vary by field, there really used to be much more of this decades ago than there is now, and while some folks feel cut-throat competition brings out the best in everyone, I'm not so sure.