He is a former LE detective that did not hesitate to call police when his gas cans were missing but yet did not call police when he smelled decomp in his daughter's car. Adding to that, his daughter had not told him that the car had been abandoned. He had spoken to his daughter but had not spoken to his granddaughter. There are several reasons why he should have raised an eyebrow at that point and called police.
For all he knew at that point (if he knew nothing at all), both Casey and Caylee could have been in trouble. I would have known that something was seriously wrong. I would have never suspected my daughter had killed my granddaughter, but I would have thought that both of them were in serious danger at that point and I would have called the police. But then again, I worry about my 23 year old daughter when she is 20 minutes late, she's always early...so it's out of her character to be late.