G'day Mike.
I just came back from holidays on the coast. I had the opportunity to test that very question. I hunted the same piece of beach frequently over a 10 day period.On the first night, I pulled up $18 with the Explorer. On the second night, I pulled up $10 or $12 with the X-Terra. By the time I hunted it with the Ace on the third night, I only found $2! The next 2 nights yielded the same result, with both the X-Terra and Explorer. I let it go for a couple of days and used the Explorer again and pulled up another $10. On the second last night, I hunted it with the Ace to only find $2. Thinking that The Ace may not have been doing so well, I immediately swapped over detectors and used the Explorer over the same part of the beach and found a 20 cent piece and 2 ten cent pieces in one spot, and a 20 cent piece in one other spot! I spoke with a local lady who comes down to the beach almost every day, and she told me that the local detectorists were all away at the moment, so I was the only person hitting this particular beach, with any regularity. There were 2 others that had hunted it once only each in that time (that I'm aware of.) The beach gets groomed every morning.
My conclusions were these. 1) Once you have hit the same piece of beach twice in quick succession, you've pretty well found most of what there is to find. 2) Those grooming machines don't seem to pick up too many coins.
I hope this throws some more light on the subject. Hopefully, those regular beach hunters can give some more insight into this.
Mick Evans.