Just to be speculative, as it is a sort of whodunnit, various things added together for me
But when you take
Colborn calling in the plates two days early and looking like he absolutely shit himself on the stand when the lawyers questioned him about his call
The boyfriend and the brother seeming sketchy about being asked if they had ever been on the property before
The boyfriend and the brother who admitted to having access to the voicemails - and then one (or more going missing)
The brother talking about the grieving process last days, months, even years while she hadn't been confirmed dead then correcting himself to mention "but hopefully we carry on with her in our lifes".
The likelyhood of the key being planted by Lenk at the the minimum.
The woman being given the camera and being told to go and look basically starting where the car was.
I can see a situation where there was some collusion and some corruption and yet Steven Avery may have still committed the crime. The lawyer in his interviews won't say he believes Avery innocent, only that he believes that the police on believing they had their man, tried to make the case even more water tight. I can also see a situation where one of the other Avery's committed the murder as they had the opportunity and two of them managed to alibi each other. This could have been on a sexual basis, or to frame Steven Avery.
So, based on the sketchiness of the boyfriend and the brother in their interview with the media (purely regarding whether they went onto the property, watch it again it looks really dodgy) and the way in which they clearly were deciding to play detective themselves from the offset - by hacking into her voicemails and phone records rather than letting the police do all of this. I wouldn't at all be surprised if they had left a voicemail on her phone saying something along the lines of "We are on our way to the last place we believe you went, if you get this message please call us so we know you are ok". They turn up to the property in the cover of darkness shortly after midnight or so from the entrance that the lady was later told to check out. They call it into the police who arrive onto the property later that night, nearer to the time Colborn calls in the plates. Colborn calls in the plates- it's the right car and he's looking right at it. He tells the boyfriend and the brother that he already knows who the guilty party is, but the case might not get very far if he's already discovered the car by coming onto the property without a warrant and tells them to leave it with him, they give him the spare key. He calls Lenk who along with Colborn more than likely already believes Steven Avery to be the perp, and also has motive in the tune of 36 million dollars to make sure it is him. They then set in motion a way in which they can ensure a conviction- they already have the car, the key and Lenk knows he has access to Steven Avery's blood and they have time in which to get some of this sorted before the car is 'found'.
It allows for them to have stitched him up, while the crime still took place on the Avery property, whether Steven or another.
Now what this doesn't account for is Halbach's body - if it wasn't discovered that night, then what's to stop the brother and the ex boyfriend to go knocking on doors and going mental to see if she is still alive. If it was discovered, then there is no chance in hell that the brother and ex boyfriend are going to allow the police to burn it simply to make sure they get a conviction.
I can only see it being
1) Boyfriend and Brother discover car - call police - plan to frame set in motion
2) Colborn discovers car and body - plan to frame set in motion - pretty extreme to imagine them burning it, but desperate people can do desperate things - being a police officer does not preclude you from this in my eyes
and the most simple which can often be the correct answer
3) Car is found legitimately - plan to frame set in motion.
The only problem with 3 is, I simply cannot believe that they managed to get rid of all the DNA evidence from everywhere else and then just left the car in a really easy area to find hidden like that. Unless of course, they actually thought that by removing the DNA evidence and getting rid of the plates - the vehicle wouldn't be suspected of being hers (but surely they'd have found her DNA in it?).
The whole thing is odd, the timing of the murder as well is absolutely incredible, it's literally days after they are deposed. I don't think we'll ever find out either.
For what it's worth, I believe they found bleach on one of Brendan's items of clothing, and he admits in one of his interviews (not sure if this was coerced or not) to cleaning something red/dark red in the garage for Steven.
It seems stupid for Steven to commit that sort of act after already being in prison, but it's possible he did, perhaps thinking that all his attendance at these events and being a poster boy for lack of justice meant that he was now beyond the law - but I don't believe it was proven beyond a reasonable doubt for me based on the evidence I have seen and if I was on that jury I wouldn't have convicted him.
