Each time you took your car to have the oil serviced the gravel guard was removed in order to get to the drain plug. Believe it or not every garage I have ever worked in has a collection of these gravel guards somewhere in a corner or out back of the shop... not because we don't think they are important, but many times we simply forget to put them back on after servicing the oil. I have no idea why this happens as every part of the car has a function but maybe subconsciously we just hate how much of an inconvenience they are to the tech changing the oil... also 90% of the time when the tech is taking the guard off, half of the fasteners are missing and the other half are broken and being held on by zip ties... I would not be surprised if they end up falling off while driving...