In the lower right corner of my Gmail page, there a link to see the access history and activities for my account. In the list, I discovered an entry of access from "Authorized Application (427071021612.apps.googleusercontent.com)" and its location is from Virginia. How to find out what really is that?
Go to https://myaccount.google.com/security#connectedapps and under Apps connected to your account, select Manage Apps and go through the list. There is no good way to interpret the code as what which specific app.
If you suspect something is out of whack, go through the Google Security Checkup, change your password, and enable two-step authentication if it isn't already enabled.
I found that it was linked to a work iphone that has not only my work emails on it, but my personal Gmail as well. If you have more than one device (tablet, laptop, smartphone, etc.), and they are all linked to your Google account, you will find this access information under that "details" for your Gmail account. I think it's kind of neat that Google informs us of that! It's a great security feature on their part!