Notifications and logging
If you use sign in as user to access a team member's account, you'll have the option to notify your team member that you have accessed their account.
Team member / admin notifications
If you click Change notification settings in the notifications window, you'll have the option to select who receives a notification that you have signed in as a team member. These options will vary slightly, depending upon whether your account has one or multiple team admins.
If you select The member and activity log (recommended), the team member will receive an email notification that you have accessed their account.