He didn't know until that exact moment. When he got close enough for him to smell it on his breath.
The scent of week-old spunk, and with his super smelling he could detect it had been four days since the sandwich star's last encounter.
And with dawning horror did he realize how old the person it had come from was. And so he resolved to do something about it.
For you see, HE was the "kid" that monster thought he was talking to when he got caught.