Hello, in adminCP you have a feature to login as the user, to see if there's a problem or what not.

But the user will know you logged in as him ?

Thank you.

Well, there is no message telling them. But they might be logged out since you created a new session and every activity you perform will show on there account as if they had done it themselves, e.g. a topic opened by you will be shown as marked read for them. 

I use that feature quite a lot.

If you log in as the user,  the date for the last logging is not changed, ( for now, it could change in the future )
Also, you do not appear as logged in even if you use any widget or block to track online users.

So unless you do something like reading messages or replying, nobody should know.