Neither do I! That voice mail message was garbled though so I reckon she told him about the plaque at the start of it. We'll get a flashback where she's leaving it and she'll have told him the truth about where she was.
Toadies going to be insufferable now though, he was already moping around when he was just thinking about shagging her now he actually has he's going to be intolerably sulky due to guilt.
This wouldn't be happening if Karl and Susan weren't off galavanting!