Gmail oddity

I discovered something very odd a few days ago. I went into a Yahoo account I have only for a specific purpose and discovered there an email from a friend. Since I've never given my Yahoo address to any friend I'm baffled. She says it was sent to my Gmail address. I have it as a recovery address with Gmail (having no third account) and with EE, which is my ISP.
Does anyone have any insight please? I've asked Gmail through their forum but with no result.

The complete delivery path is contained in the message headers, so you should see in there if it went via gmail.

https://help.yahoo.com/kb/SLN22026.html

Did you also receive same email in GMail?

Have you got GMail as a linked a/c in Yahoo ("Add another mailbox")?

No, it didn't arrive in my Gmail Inbox. The Yahoo address is only in my account, along with my mobile phone number, for recovery purposes. Weird, eh?

Have you checked the route it took to you, as suggested by WWWombat?

Thank you for your interest and suggestions but I'm afraid I have no idea what WWWombat was on about or how to go about it. I'm not, at this point, worried about that email and fortuitously, it wasn't an important one. I think it's a problem for Gmail to answer as any email sent to a Gmail address shouldn't arrive in a Yahoo account..

Thank you. I deleted this email but if it happens again, I'll check.