Thought I would just see if anyone else has had any experience of something like this.
Yesterday my girl had her first pup at 11.40 all went well. Second pup came at 13.50, again all went well. I knew she was having a small litter so after this we took her outside for a wee and had a good feel and other than the uterus horns nothing could be felt. After her feeding the pups again we washed her back end and again had a good feel, nothing to feel. She settled down with her babies and I had another good feel both sides and nothing to feel. Then between 15.30 and 21.30 had 4 big bowls of food, fed and looked after her babies and slept well. She did do some panting which is normal. I went to sleep beside her box at about 22.00. Every so often peeping in the box to check all was well, which I was pleased to find happy pups and mum.
Later in the night I was aware that I could hear her licking, more than the cleaning of pups and thought she must be having a drink. This went on so I thought I should open my eyes and check. To my shock I could see what looked like some blood and she appeared to be eating something! Fearing the worst I jumped up, knocking the water bowl all over the place, only to find she had delivered puppy number 3! She had de-bagged him and was eating the placenta! Pup was squiggling around so I quickly dried him and gave him back. He latched onto mum and fed away with gusto! He was strong and the biggest pup of the 3, not even gasping or full of any gunk!! He was born over 12 hours after the last one! Where she had been hiding him goodness knows, must have been in her ribcage, she is a Lhasa Apso so not a big breed. I have had many whelpings in my time but never had this! Just proves you learn something every day and nature is a wonderful thing!
All pups are doing well and she is being a wonderful first time mummy.

Friends of mine had a very similar experience last year. Their bitch was scanned carrying two pups, the scanner is a very experienced sheep scanner, she's never wrong. She was a bit surprised though by how far away the pups were from each other.  4 weeks later Mum whelped the first pup, nothing for 6 hours so they took her to the vets, the vet scanned her and said remaining pup was fine, no need for a section. So they went home and waited and the other pup was born the next day, absolutely fine.

These pups were sired by my boy, he of the famous long ties, (1 - 2 1/2 hours!) I think the genes are laid back... no worries, no rush

I had a Long Coat Chihuahua bitch who didn't look pregnant.  I never scan, and always assume pregnant.  Mine usually only go 56 days before whelping, and on the day I was thinking she hadn't taken.  She was laid next to me on the sofa.  I was reading and gently stroking her tummy.  All of a sudden there was a big whoosh as 2 came down from under her ribcage, and she gave birth really easy.

I had similar.  As my bitch wasn't proceeding with her whelping (mine did tend to go to secondary inertia) I took her across to my then vet who was some distance away (in Canada) and he looked, and gave her an oxytocin inj. which produced puppy number one.   As he lived over the Practice, I opted to stay put in one of the exam. rooms and camped down for the night, giving it a couple of hours if nothing happened at which point I rang him to get him down.   I did this with all that litter and by morning, when he finally came down, he pronounced her to be empty, and sent me and pups home.  

I was getting myself sorted out, leaving my husband (who'd had a good night's sleep!!) to watch.   He yelled that there was another puppy arriving.  That puppy must have been in the birth canal, out of reach of my vet's fingers, or palpation.   So it can happen that one would be 'hidden' - in the birth canal.   I was really happy because at that point she'd only had the one bitch (and 4 males) and she was a doubtful as she'd been very slow coming through and I feared for her.  Quite rightly as she'd suffered with oxygen starvation and by 18 weeks, we had to let her go as she was brain affected.  Happily the delayed pup was a good-un which I kept and showed to her Championship out there.  She featured in all my ongoing breeding programme.