Review of 500: Tobi was one of the 4 ANBU
Chapter 500 has some vital information for review, when you keep in mind the question: How did Tobi know that Kushina was giving birth?
On Page 10, we are told that only Biwako, Taji, and Minato will be inside with Kushina. Koharu and Homura said that the event must be top secret. Presumably, this can mean they didn't tell anyone else. We know those two, Biwako, Taji, and Minato aren't Tobi. We're also told that there will be the Third's personal ANBU stationed outside.
On Page 11, Kushina, after being scolded by Koharu, presumably didn't tell anyone of her mission. Koharu also advised her to not scream her head off, presumably because she did not even want to tip off the ANBU outside.
On Page 12, Kushina can clearly be heard giving birth from outside the fortification. There are 4 ANBU right outside the cave and one a little ways off.
On Page 14, three ANBU right outside the entrance can be seen dead at Tobi's feet.
Where was the corpse of the fourth ANBU?
I think he donned a cloak and a different mask so that he would not be recognized when he came up against Minato, Kushina, etc. and then barged on in with his sharingan. He heard the screams, surmised what was happening, and put his plan into action.