Something I do wonder is that it was previously stated he wasn’t initially known about, but was discovered some time later. He doesn’t really appear to be in hiding. He’s in a fairly public location. He probably saw a few people at some point over the few days before the hive mind activated, but when they all joined they didn’t notice his absence. So what happened that alerted them?

They may have assumed he was amongst the dead until they discovered him hiding in his building.