Applying Ockham's Razor here, surely the most probable option is that nothing actually happened, and for whatever reason he's simply misremembering it? Maybe he was going to put the poster up but didn't, maybe it fell off but he forgot about it, etc.

And if this seems implausible, consider -- how many times have you found yourself saying things like "I could have sworn I left the keys right there", and then finding them somewhere else? Is it more likely that you misremembered, or that they were moved by ninja-like FBI agents?