I'm pretty sure I had several of them left from previous event (2 years ago?). I cant find them now, even with "Find my item" function. Are they silently removed because of next week Yeti event? Can anyone else check if still have them?

Those might have been rolled up with a lot of other old event seals under "[Event] Reward Seal" in the Manage Currency UI. The patch notes from when they did that would list them.

Indeed it was: https://www.naeu.playblackdesert.com/en-US/News/Detail?groupContentNo=7028

Thanks for pointing this out!