I’ve now had two users experience this. They upgraded with SmartUpgrade (7.0.2 to 8.5.1) and the mail file was upgraded with a policy. At some point soon after that, some text that they had typed into a reply was appended to their Signature. So, then any email after that had this text in it.
It was fixed by going to the Signature preferences and removing the extra text, but how would that happen in the first place? Two users didn’t go in and change their signatures inadvertently. Are there any new shortcuts to making selected text part of your signature?