Missing Wheel

On Thursday after Pilates, I headed to the mall to get the kids feet measured. Graham's shoes, while his feet have been measured within the last 2 months, were starting to be hard to get on. I pull into the mall parking lot and get a killer spot, I quick look at the clock and realize the mall is just opening. That is the advantage, I guess, of getting up at 7am to go to a Pilates class. It is 10am and I feel like it is 1pm. I opened Claire's door, open the trunk, pull the stroller out and open it up and it falls on it's side. I look down and it is missing a wheel. Claire says, where's the wheel, I say I don't know. I quickly look around, analyze the situation, realize this isn't a quick fix and calmly ask her to get back into the car please. She says why, and I say I need to figure out what what is going on and I can't do it with her in the parking lot, at the same time Graham starts yelling, Mama! Mama! MaaaaaMa! Ok, so I have a stroller with 3 wheels, I squat down, no wheel in the parking lot, none under cars. I pull everything out of the trunk, no wheel. Hmmmm, well this is going to be a different trip to the mall than I expected. At this moment I pause, noting that the situation, had it occurred a year and a half earlier, would have ended very differently. As I pull the umbrella stroller out, I am thankful that this snag in my day did not turn me into a watering pot. We walk to the mall, Claire in the stroller, Graham walking next to us. Both of their feet measured the same, so their shoes are fine. We played for a few minutes then headed home for Graham's nap. No sign of the missing wheel, I looked in the garage when we got home and Warren will look in his trunk today. After that I guess I'll need to call the manufacturer about getting a replacement wheel. How does one lose a wheel off of a stroller? More to the point, how do you lose one and not know it?