^ Maybe he waits for some idiot like me to pick up all the bottle tops in the daytime so all his 'beeps' may be something more worthwhile?
The tide times change. Maybe it's better to search at low tide when the last high tide has disturbed the sediment and exposed new 'treasure'. It could have been coincidental that low tide was also in the evening at that time? This has given me a thought for tomorrow's treasure hunting!