Not all good finds are two way repeatable signals

These two Spanish military buckles are Treasure Coast of Florida finds from a few years ago, found at the same beach a month apart.

Check out the file marks on the one buckle and the fastener pin still attached to the other, not bad condition for stuff from the 1700s recovered off a beach. 
I was using a pulse induction metal detector when I recovered these artifacts, searching for deep targets that often do not respond with a classic two way repeatable signal response. 
As with anything metal detecting related, you have to use the equipment and search techniques to suit the site and what you are searching for.
The only thing that gave these Spanish artifacts away was a slight lowering of the threshold (Metal detector back ground noise) so if I was not paying attention I would have missed them. 
I often dig all target responses when I know a site potentially holds what I am searching for, even a slight break, lowering or raising of my metal detector threshold.  
Very deep targets on the edge of metal detection range respond quite differently to shallower targets which are easy for a metal detector to identify.
Some of my favorite old finds were "Knee deepers" stuff that many beach hunters would miss if only stopping to dig two way repeatable signals.
Threshold sharpening skills are obviously best learned when you have a decent threshold level, I know many beach and water hunters prefer to have a barely audible threshold which is ok for tourist type beach hunting but it is not the best threshold to use when searching for old stuff.
A beach hunter searching for old coins and artifacts needs to maintain an audible metal detector threshold, so you can hear subtle and not so subtle changes in the metal detector background noise.
You do not always hear deep targets, unless you are paying good attention to your metal detector threshold.
I always turn my metal detector threshold a little higher when searching for gold chains after recovering a gold pendant at a tourist beach.
Then I dig any anomaly in the threshold, just in case one of them is the gold chain the gold pendant was attached to.
Good practice at trashy tourist beaches for artifact hunting, you will be surprised how many things you recover from an area you thought you had searched thoroughly. 
Save the two way repeatable signal digging for tourist beaches, where tourist bling is the main target.