When someone is gone sick from school often/for a long period of time, and you don't know what happened to them. Under the bleachers means it is possible that they have bird flu, are in a spinal halo, and are taped under the high school football bleachers. You just don't know.
"Where's Johnny today? He was supposed to give me his phone number."
"He's been gone for a few days, he must be under the bleachers"