When the Vogons arrive at Earth, we know that Ford's Sub-Etha Sens-O-Matic was going off for the first time in 15 years.
From Book 1 Chapter 3:
He knew exactly what was happening and had known ever since his Sub-Etha Sens-O-Matic had started winking ... It was what he had waited for all these years ...
When Arthur and Ford board the Heart of Gold we learn that Zaphod Beeblebrox visited the Earth approximately 6 months before the start of the book.
So how come Ford hadn't detected Beeblebrox's visit?