In episode 21, Misa Misa killed Kaneboshi with the help of Rem. Why didn't anyone question how she did it? Or did everyone assumed Higuchi did it?
At this point of the anime, L et Light don't know who is Kira, they only know that he is working at Yotsuba.
First of all, when she's passing an interview with the members of Yotsuba, when going at toilets, Rem make Misa touch a piece of the Death Note so Misa can see her and reveals her who is Kira.
So only Misa knows who is Kira at this moment, so there's no reason why they would have placed micros or cameras on Higuchi's car.
Then, Misa decides to make Kira reveal himself. She swaps her clothes with a friend so Mogi can't stop her. The date with Higuchi was not planned by the detective team so they couldn't put recording devices on the car.
In the same episode, when Matsuda says "We should maybe put a micro on her", implying they don't usually do it.
So what's happening in the car is not recorded by L & Light.
At 20m30, we can see that Misa activates recording device :
There's even a little bip noise indicating that it is starting to record.
The only part that is recorded is that
Higuchi will stop killing people in order to prove that he is Kira so he can marry Misa.
The death of Kaneboshi isn't mentionned. As said earlier in the episode, Kaneboshi was known to be a bad guy. His death isn't unusual and there's no reason why it would have been noted by the detective team.