Attempting to locate a mysterious alert

So, in my farm admin mailbox we receive regularly "out of the office" mail messages and even an occasional Cc of a message that is sent to others from a task, etc.

We also get, occasionally, Exchange bounce messages when a user's mailbox is full, or a user has left the company, etc.

There is one specific message of this latter type that is puzzling me. It is indeed being sent to a user no longer at the company.

However, when I run a PowerShell script that lists out all alerts titles and the users to whom the alert is associated, I see neither the title of the alert in this bounce message, nor the user.

In the header of the message there are several mail headers that indicate this is an alert:

X-AlertTitle: =?utf-8?B?RGFpbHkgU3VtbWFyeSBOb3RpZmljYXRpb24=?=

X-AlertId: {264BDF97-930B-40CC-A1DA-39630DFAB460}:{00000000-0001-0001-0001-000000000000}

X-AlertWebUrl: =?utf-8?B?aHR0cDovL2Nhc3BvcnQvcGVyc29uYWwva2xiOTM=?=

X-AlertServerType: STS

Subject: T14

So, are there alerts which would not appear when a farm admin runs through each of the farn's get-spwebapplication object's Sites > AllWebs > Alerts ? The number of alerts that were returned looks about right to me.