I find this quote in particular, quite telling. It indicates that when he began acting suspiciously, she began to suspect his involvement.
You know how we hear cases of women saying to their friends “If something happens to me, my husband did it?”
Going by this statement by NA, something like that didn’t occur, and her suspicions came later, as a direct consequence of him behaving strangely.
Good point! To NUA's credit, she was very aware of how things looked and how he was acting.
I hope I have a friend like her if I ever go missing.
However, I have to say, the evidence was pretty stacked up against CW being responsible at that point:
1. NUA observed SW enter the house and close the door around 2 AM, so she knew SW got home.
2. The back sliding door was locked and blocked so SW couldn't have gone out that way
3. The top latch on the front door meant meant SW couldn't have gone out that way.
4. NA had a key to the front door and knew the top latch was only used at night.
5. The neighbor's camera on the driveway made it impossible that SW and the girls left the house unseen.
6. CW was the only one seen exiting the house out the only exit possible (on camera/out the garage).
7. CW was seen on camera loading things into his truck.
8. SW left her purse behind.
9. SW left her phone behind (and it was in the upstairs sofa).
10 SW left her children's vital medications (epipens) behind.
11. SW left her car behind (with the keys).
12. SW left her children's car seats behind (in her car).
13. SW's shoes were by the front door.
14. SW's suitcase from the trip was by the stairs-- she had not unpacked/nor taken it with her.
15. The neighbors had not seen nor heard SW nor her girls that morning.
16. The stripped beds were not how SW would have left her house.
17. No communication from SW was out of the ordinary.
18. Missing a doctors appointment was out of the ordinary for SW.
19. There were no signs of a break in.
20. There was nothing of value missing from the house (except the people).
21. CW
claimed to NUA he had texted SW that morning without response but he wasn't concerned about it.
22. CW mentioned a playdate but had no names and no times for that.
23. CW did not rush home the first time NUA showed up at the door saying SW and the girls were not answering. He showed no concern over that.
24. The next time NUA calls, CW suddenly brings up separation (NUA's reaction was that's not my business/I just want to find SW and the girls).
25. CW only came home when informed the police were called.
26. CW further delayed getting into the house by not permitting police to break in/not giving the garage key code.
27. CW said the keypad to enter the garage was broken.
28. CW made the police and NAU wait until he got there before letting them in.
It all added up for NAU quickly and these are just the things we KNOW about.
I imagine there are other things we don't know that she may have observed that first day or we might find out about later such as:
How did CW appear when he got home?
What else did he say?
Did he mention his blowing through red lights to the police?
Does his truck's GPS show he really sped and ran red lights?
Where does his truck's GPS show he came from?
What was the first thing CW did when he arrived?
Did CW tightly control their entry to the house/Ask them to use a specific door?
Did CW yell for Shanann as if he was worried?
Did CW disappear from their sight in the house at anytime?
Did CW eat lunch that day despite being informed of his wife not answering the door?
Did CW mention food or getting dinner later after the police had looked through the house?
Was there any evidence SW had taken her morning Thrive pills, made the usual morning shake or put on a new patch? Any wrappers in the trash?
Was the Thrive patch from the previous day found?
Were there any breakfast dishes in the sink? How many?
Anything indicating she had fed the girls that morning? Or just dishes for one (CW)?
Any other things he didn't clean up that SW normally would have?
Was the garage keypad really broken?
Was the home security system disabled or working? Did it show everytime a window or door opened?
Was Dieter where he normally would be if everyone had left the house?
Were the girl's toothbrushes checked and were they dry indicating they hadn't been used that morning?
Were bathroom towels dry (or absent) indicating no one had washed up?
Did either of the girls still wear pull ups to sleep? What was in their bathroom trash?
What clothes were in the clothes hampers?
Were the clothes SW was wearing the previous day found?
Were the clothes the girls were wearing to the party found?
Could CW describe what the girls wore to bed and were their pajamas found?
Was there any evidence in SW's bathroom trash showing she had removed her make-up the previous night to get ready for bed?
Where was the jewelry she had on the previous night? Was it found in the usual place she would put it before going to sleep or was it missing?
How was SW's phone located and who found it first?
Did police look through her phone while NUA was there?
Did they look through SW's purse and see anything else amiss with it?
How many people did NUA call asking if they had heard from SW before CW made it home?
(I bet she called everyone she could think of and no one had talked to SW)
And all this BEFORE the lying porch interviews.
Those interviews are a huge boon for the prosecution but everything NAU and the first responding officers observed the very first day would be enough to point the finger at CW alone.
JMO.
). 
I think there's more to the story there almost always is. 
