Non conventional target signals.

Many beach and shallow water hunters do not realize they are missing clear signals and walking over potentially valuable targets. 
I am talking about signals within your metal detector threshold (background noise), this is especially important when targets are very few and far between. 
A slight raise or lowering of the threshold could be a deep target out of normal discrimination range, so too could a short break or null in the threshold. 
Time spent using your metal detector will help you to tell the difference between a threshold nulling over a ferrous object, and a threshold that has been interrupted or changed by a deep non ferrous target.
On beaches known for old shipwreck artifacts, or out searching in deeper water away from shore on tourist beaches, I rely on my threshold more than anything else.
I have found many large pieces of platinum and gold jewelry which were detected and successfully recovered after I stopped to investigate a slight raise or lowering in my metal detector threshold.  
I am quite sure many beach and shallow water hunters only stop to dig two way repeatable signals, even in areas where targets are scarce. 
It goes without saying, if you are running with a silent threshold, you are not going to hear any deep targets on the edge of your metal detectors normal metal detection range. 
If you search using an all metal mode, the easiest signals to identify are double blips from hairpins or thin sparkler wire type ferrous targets, and solid two way repeatable signals from jewelry, coins, etc.
The hardest signals to identify often turn out to be the best finds, they tend to be deep targets found by listening to slight changes in the threshold.    
This iron canister shot was found using this same technique at an inland site, only a slight  lowering of my metal detector threshold alerted me to the very deep target.