I agree, JB. It is a bit odd. Perhaps the "death smell" portion of the last 911 call was not relayed to the officers since they had already been dispatched, and was only noticed when all the 911 reports were written up and combined with YM's notes of the interviews that night and the Anthony's signed reports.
Perhaps the Anthonys admitted the car smell that evening but attempted to downplay it by repeating the line of the employee at Johnson's when he opened the trunk and said "oh here's what's causing that horrible smell" and threw the bag of garbage over the fence . GA said he and CA had turned to each other when they brought the car home and "agreed" it was pizza that had caused the smell.
Even if that is what they told LE when they arrived, LE was still on the ball enough to go back to Johnson's Towing and get that garbage bag that had been thrown over the fence that very night. I just don't think it may have initially connected the smell in the car with a crime scene yet, or if they did, they may not have wanted to make that connection in front of the grandparents until they did a little more investigation, either out of sensitivity or simply suspicion. They knew the car had been driven home by the grandparents regardless of the smell and enough time had passed for someone to interfere with maintaining a pristine condition of the trunk. Their initial priority that evening may have been simply trying to locate the baby and put the car smell second until the garbage story could be corroborated or until it became clear that, if Caylee could not be located, that made the car smell a more ominous sign.
It's apparent that after the Imaginanny Wild Goose Chase that night however, LE sent a homicide detective along on the Universal Tour the next day, the 16th, when KC was finally cornered in that office and questioned, which is also when they impounded the car after they were unable to get KC to confess to anything, particularly that she may have had a hand in Caylee's "disappearance".
Perhaps at that point, LE was less suspicious of the Anthony family and any attempt they might make at obstruction of justice (ie: cleaning the trunk) and more focused on KC, who they were keeping a close eye on and had in their custody for all but a few hours after their arrival the previous evening. I'll bet they wish they had impounded it earlier at this point, but may not have thought that KC would have been as resistant to questioning at the time.
They may also have either realized that the trunk had been compromised anyway, as it had been opened and gone through by the grandparents, and they were not necessarily in any hurry to impound it. If an attempt had been made to clean up evidence, it would likely be very apparent once it was examined by experts.
I'm sure LE may now wish they had done certain things differently, but I was there when the recordings of the "Universal" interviews came out, and I was shocked that some local channels (and commenters) actually were criticizing LE for coming down too hard on her in that interrogation. I couldn't believe that anyone could think LE was less than professional or were acting too rough on her. It made me think that, for some reason, LE knew they had to handle this with kid gloves from the get-go. JMHO.