It could be my mistake, but I thought that if I book wasn't mailed in 5 days (never acknowledged) that their account went on hold?  A person didn't mail me the book because the request timed out, but their account is still active.

I believe it goes on hold if they have too many requests time out.

From what I can see, this person hasn't been on since 2014.  Or at least hasn't WL'd or posted books.  Shocking that it hasn't done so before now lol

When this has happened to me the hold does not happen until some time late at night on the day of the 5th day.

No, your account is not suspended if you miss one request.  It could be that the person went on vacation and forgot to put their account on hold, which means they wouldn't accept the book; or they could be ill.  You can't punish someone for being ill.  If you do it several times, then probably.  What does happen, though, is the book is removed from your shelf and you have to repost it.

There is a secret percentage before the account is put on hold. It is more than one request. More than one action.

Failing to mark a book as received usually triggers us objecting, triggering a manual review. Failing to accept an order just causes the computer to respond. It takes several computer responses before the account is flagged for a human review.