I just got a horrifying e-mail from Google that led to a webpage stating that there was a "New sign-in on LG Nexus 5". I immediately scrambled to change all my passwords to protect myself because I didn't recognize that device, but how could I find out more about it? I was in fact logging into my account recently using a different device, just not a Nexus.
I found out later that in fact the perpetrator was me and that Google had incorrectly identified a Samsung Galaxy Note 4 device of mine as an LG Nexus 5. To confirm this, in the "Security alert" e-mail from email@example.com, I clicked "CHECK ACTIVITY", which brought up a webpage (the one titled, "New sign-in on LG Nexus 5") where I saw the alleged perpetrator's IPv6 address and the approximate time of the alleged breach. This perfectly matched my own login activity timeframe and my Note 4's IPv6 address as found in Settings | About phone | Status, under the heading, "IP address".
So it appears Google does in fact misidentify certain hardware models when informing you about potential security breaches, and that it's worth investigating the source of each security alert in case it's actually a false alarm like this.
I also found the following post relevant and useful: Google Account Compromised - Possible Investigation?