I found it but then I closed out of the tab before I even realized it was there. So I missed it completely. :( Will it show up again, or was this my only chance?

It'll show up again. :) I know I missed a few due to the flowers (I would open a tab and not notice the bug, and open another tab... which would then replace the bug with a flower). The bug eventually pops up again.

So I noticed, I found it again later.