Unseen mail rules exist?

Most (if not all) incoming emails with/about invoices are automatically marked by Mail as read and moved to an archive box.

This is obviously embarrassing as it means I'll always be late in paying those invoices.

As far as I'm aware this does not happen to any mails with other subjects.

I've repeatedly looked for any mechanism that does this and I can't find any rules that would cause this.

Are there "unseen rules" - would Mail simply copy the normal actions of reading and moving the mails?

Can there be another cause for this?

All software is up to date; this is not new, it's been going on for at least two years.

It's not just one account: I receive invoices on a business address, I receive Apple invoices on an Apple address and both addresses show this issue.

A MacBook and a Mini look at this Mail set (as well as an iPhone); I've checked both sets of rules (yes, even though I know they synchronise. How desperate can you get?).

I also can't tell if this behaviour is specifically caused by just one machine or by both.

All help much appreciated!

Cheers,

Bob