Hey Uma,
Assuming you have no other hints about where it is or what smart campaigns/engagement programs are sending the emails, you can go to Design Studio > Emails and you can see all of the emails that exist in your instance. You can sort by "last modified" and scroll down the bottom and work your way up. That could be tedious depending on how many emails you have. When you're logged in, add "#EM0A1ZN14" to your URL and it should go to this section (e.g., https://app-sjg.marketo.com/#SC1642A1 for me).
If it's triggered, you can work backward to reproduce the trigger.
If it's scheduled, you can do an email performance report in your Analytics section, limit it to as small as possible for the time the email was delivered, and work through that list. That'll get rid of a lot of the noise from existing-but-not-in-use emails. Then work backward by finding each of those emails in your instance and see which one is the culprit.
Really, the most efficient method of finding the email is going to depend on your instance. If you have thousands of emails, that are unused but stil exist, do an email performance report with a constrained time period. You could cross-check with the "last date updated" in all emails to get a prioritized list of emails to check.
(Or as another option to throw into the mix, you could work backward by email template, if you can receive the email and distinguish which email template is being used.)
Best,
Edward Unthank | Founder, Etumos