I was driven up the wall by this.
turned out it was my oyster card (London underground NFC card). The phone was in a leather wallet style case, which had a slot to hold credit cards ( hence oyster card) .
So open case, no problem. Fold it back (eg in sat navigation holder in car) it is ALMOST close enough to trigger NFC "boing" .... go over a very small bump .... It moves closer, "boing". Take it , no sounds, fold back case to investigate .... "boing" ... clue was in comment above ... many thanks