Next time you drop him off, I would have a chat to the carer and mention what you saw. Given that he has been unhappy about you leaving him there, I would also mention to the carer that you are concerned this may be happening often as your DS1 is unhappy about you leaving him and could they keep an eye on him.

As a Mum, it is so hard not to react when we see these things. Like you have done, it's worth thinking about what you want to say rather than react immeadiately in the heat of the moment