She first said it when they were hiding behind the tree where Yui was counting. She was trying to say 灯台下暗し(とうだいもとくらし) which means “it is darkest under the lamp post; it’s hard to see what is under your nose”. That phrase would have made sense given the situation.
But it’s a complicated phrase and she’s only young so she’s said it wrong, saying something that doesn’t make any sense.
For what it’s worth the Tokyo University campus is just the other side of Ueno Park.
I read this as - That degree of surplus! Where is that? i.e. Where do I find this place which has an excess of people with streetpass?
