I had 8 chicks in my brooder and only two of them ever "escaped" far enough to be out on the bathroom floor. Most of them just perched on the sides, jumping there and back down after a while. The two that got out were at different times; one couldn't get back in and settled down next to the brooder as close to the light end as she could get. Luckily, by this time they didn't need it warmer than 80 degrees, and the bathroom was like a sauna, so she was okay until I got home from work.
The other one was one of the first to perch on the rim of the brooder. I'd seen evidence of her checking out the bathroom counter and sink, and toilet. I opened the bathroom door when I got home from work one day, and she was in the middle of the floor investigating the bath mat. She peeped at me, and scurried back to the brooder, hopped up the the rim, and back into it.
I've got a different brooder this time, with sides just a tad lower, so I may have to put some hardware cloth over it at some point. The new chicks may be able to get out sooner than at "80 degrees warm."