Thinking it was shop purchase notification, I accidentally closed out a sidebar event notification that someone gave me a sticker. I want to thank them for it but I'm not sure who it was! Is there any way to see past event notifications once you've closed them or are they gone for good?

Unfortunately, not once they're deleted. If you have another tab open, it might still have the notification on it. You can try the events page but IME it still updates pretty fast. And for forum pings, there's also the ping history but no entire notification history

~ aww bummer. Oh well, thanks for letting me know there's an events page though :)

your best bet is to post a thank you on the Gift bragging forum. Either on the big anon thank you board, or make your own.