It happens sometimes that somebody (likely) meant to reply all in an email chain, but (likely) accidentally just replied to the last person that sent the email. What is a polite way to inform them of this/ask if they actually intended to reply all?
I'm assuming, in this case, you were the last sender and by clicking the "reply" button, you are the only one who got that email. Then, One way could be, reply to the email which was only sent to you and inform them about the mistake like
Hey, did you mean to reply-to-all?
That should do just fine, no need to overthink it.
Considering another case:
Where you were not the last sender, you were missed in the email whereas, the intention was to have you (and others) in loop.
You may have received the update from someone else and upon "investigation" you discovered it was probably because of the mix up in "reply" vs. "reply all".
In that case, probably the best way out is to reply to the original thread and request for any update. If it was a genuine mistake from the sender, you should see the email they sent out previously to be forwarded to the entire recipient list pretty soon.
You can say something polite like:
It looks you intended this email for all of us, and I'm guessing you meant to hit Reply All?
To avoid problems like this, I would like to share a handy email habit that I learned about at a previous workplace. Here it is: when someone in the team adds new people in an email thread, they would start the email body with a sentence like this:
[CC Bob Smith, Alice Jackson]
Because changes in the CC field are very easy to miss in most email interfaces, this would point out the change and indicate that the change was intentional.
Similarly, if someone removed people from the To: or CC: fields, they would start the email body with a sentence like this:
[To Alex only]
Again, that points out the change to a reader who might have missed it, and indicates that the change was intentional.
If the people in your workplace are in the habit of using a system like this, it makes it obvious when somebody accidentally hits Reply instead of Reply All, and you can confidently add back missing email thread participants without needing to ask permission first.
It seems some people didn't get the message, let's include them now.
That way you aren't blaming anyone. It's also useful when by mistake a certain individual gets taken out from the conversation and then you can say
Just including XY in the conversation, it seems he didn't get some of the messages.
Again, not blaming anyone, even though a perceptive person will be able to see who made the mistake in the conversation chain.
What I personally do, is if I think that the original respondents would benefit from the email, I either forward the email to them or reply to the email saying something like thanks and add the other intended recipients.
No need to do anything that can be taken negatively and there is a possibility that the respondent will understand that the others would have benefited from his email.