It seems to be happening again. I picked it up this morning and it was in ES mode. All I did was change the focusing mode from S to C and back to Single. I also changed the AF from Zone to Wide. No other settings were changed.
None of the other settings seem to have automatically changed themselves apart from this, so it's weird.I don't have a lot of time to go into the menu but even if I did, I wouldn't know what to look for that might be causing the shutter to switch to ES after having been put to MS. I'll try a reset and have him use the original battery instead of switching between the non OEM one that was supplied just to rule that out.
This occured to me several times, until i found out that i accidentally configured a button for MS-ES selection.
When you touch that button, ES is selected without noticing it. It changes in the menu on the display, but it is hard to notice. So try your buttons and see if there is anything changing.
I think my button was hold accidentally for a few seconds, so it got in to configure mode. Then ES was selected, and from then on the button function causes the change from MS to ES.
(excuse me for my english, i am not a native speaker ;-)