The interior lights would have gone on if they were working and were set to go on. However, interior lights are not bright lights in most vehicles. In my vehicle the lights that come on do not light up the entire interior of the vehicle and do not illuminate areas closer to the floorboards.
I've gone out to my car on many nights to retrieve something and unless I had a bright flashlight with me I could easily miss said item. A smudge of something like blood on any of the interior surfaces I would not be able to see without extra light, as those console surfaces are very dark (dark gray).
Why didn't SA double check the SUV? Why didn't SA think he should be extra diligent? Because he didn't, simple as that. If SA had been diligent he would have gotten rid of the rifle hanging on the wall of his room. SA wasn't that diligent.
Smart people make mistakes too. Bernie Madoff anyone? Brilliant man. Got caught. Prison is filled with people smart, dumb, and in between. They don't think of everything and the average crime has something like 10 or more mistakes by the perp, ranging from statements they make to telling someone something to forensic evidence, to something on a computer or cell phone, to behaviors others notice...you name it. Many murderers do get caught. Extremely smart tech dude in my town got nailed in the murder trial of his estranged wife. He did a google search at work, on his secure work laptop, in google maps and zoomed in on the very spot where he would dump her body...12 hours later. He was considered mensa-level smart. But he didn't know everything, and the browser he was using had a bug in it and an FBI cyber task force found his google map search right down to the individual tiles zoomed in. OOPS. Busted! He eventually admitted the killing and gave up all rights to his 2 young daughters to take a 2nd degree plea.