Forgive me, but I don't have a crystal clear recollection of events, but yesterday, a Windows authentication screen popped up asking me for my Enpass credentials.
I didn't think much of it, but in retrospect, I don't recall this happening before.

Should I be concerned?

Do you store your vault in a OneDrive location?
If so, the token might be expired and needed a re-authentification.

That’s correct. If your vault is synced with OneDrive, an expired token could trigger the re-authentication prompt. It’s usually nothing to worry about, just a routine security step from Microsoft when the token expires or refreshes.