I would file it under 2), assuming it was well written rather than just a generic dashed off thank you. In my relatively brief interview experience for software

I would file it under 2), assuming it was well written rather than just a generic dashed off thank you. In my relatively brief interview experience for software developers, I have yet to get a thank you note (and I'm ok with that). I would lean toward being okay with the surprise thank you, if only on the grounds that I think we all accept that such things are discoverable on the internet and the intent was clearly non-malicious. That said, I cannot imagine ever changing my decision after receiving such a note, so I'm curious what the goal of sending it would be? Just obligation?