Invisible 'save' button - Adobe DC
 

Its gone - not greyed out but 'gone'.

The space is still there on the toolbar

If I change a document and pass the mouse over the gap, the ALT window shows 'save'.

I click on the gap and the little stopwatch flashes - and the save registers.

Biu there is no button to see - any ideas - its irritating me

.... and it has just returned (I did nothing !)

Little glitches like this are common.
Do resist the temptation to do ANYTHING!
In a few days or hours, most of them right themselves.
If not you can start investigating.

Mac

CTRL + S is also worth to give it a shot.