I mailed a book with D/C. In the details regarding the book, the last entry says "Notice Left". What does that mean

That means it probably doesn't fit in their mail box and so a notice was put in there to get the book at the post office.  

Okay, it was left two days ago, how long do they have to pick it up?

Generally they have between 14 - 21 days to pick it up.  USPS will "notify" them 3 times usually 5-7 days apart before they return the item.

USPS left one of those orange slips for the addressee to come pick up the package.

That's what was listed when my mail was on vacation hold.  The package was scanned when it was put in my bin.   I had 30 days to pick up my mail.

I just checked on the USPS tracking site. The member has until January 29th (that's 15 days)  to pick the book up. If she doesn't pick it up by then, it will be sent back to me. I'll wait this out before I ask what will happen if it's sent back to me.

I have been home all day,  and today is a holiday for the usps.  So I know that I will not be receiving any mail today.

Well,  I just checked on two separate books that were both sent with DC, and it says that "notice left"  for today at 11:30am.   Huh????  I went out to check my mailbox, and there is no notice,  no mail,  NOTHING.  

I went to usps.com  and entered both of the DC numbers, and pulled up the standard "attempted to deliver, notice left,  please schedule redelivery"  

Has this ever happened to anyone?    

Is there some postal elf secretly working today and having some fun by creating havoc? 

I have heard on postal discussion boards that there are some post offices who require their employees to scan everything as attempted, even if they are not attempted, in order not to have a bad scanning percentage.  It's improper for managers to have their employees do this, but I have heard of it happening anyway.  If it was me, I would complain.  Go to usps.com, click on the Customer Service link at the top right hand corner of the page, then on the next page scroll down to "How do i file a complaint or get general assistance?"

I receive most of my mail at a post office box and must pick up my packages and books at the window. Ive noticed that the delivery details say "notice left" several hours (or even a day) before the notice is actually placed in my box.  I've concluded that "notice left" means that a clerk has filled out the notice; it does not mean that the notice has been placed in the box. 

To further confuse things, if the mail carrier is delivering to a business address when it happens to be closed, the tracking details will say "notice left", but in fact they don't leave a notice, they just attempt redelivery the next day. 

My daughter and I do a lot of mail ordering, and it is great to be able to check the tracking information -- especially now that we have deciphered the system!