Maybe LE discovered that the Mosler had been locked (and not unlocked) since long before the crime, so there was no reason to search it. (It sounds like based on people's research there is a chance that it might have had the kind of locking device that would record when it had last been opened.)
Remember the ONLY person who says JW dropped the money in the Mosler is JW, an unreliable source of information. I wouldn't be surprised at all to find out that JW was never directed to put money in the Mosler and that's why he couldn't tell LE whether it was locked or unlocked. In fact, I think it's highly likely!
Definitely possible, but I think since JW said in one of his stories that he put the money in the car, LE would need to look inside the car to rule the story in or out. JMO.