What is the illusion the rats have?

Rats are neophobic, which means they fear new things. However, they only have a three day or so memory. So when you set out a poison bait station, they avoid it for the first few days.

The "illusion" they have after that, is that they think the box has always been there then. Because they can no longer remember a time when it wasn't. So they go in and eat the poison bait, because they no longer fear it as "new".