Your Lexus LS300 splash shield protects a number of components from the very real threat of water-based damage. From the wheel well, to the brakes, to the engine itself, the splash shield is a line of defense of sorts that may often go unnoticed, but is nonetheless relevant. Without it, you might have faulty brake, and worse-a broken down engine. Because it is hidden from view, it's somewhat difficult to tell when it has any problems at all. There are a few things you can look-and listen-out for to make sure you catch the trouble before it grows.
Water tends to make a very specific sloshing sound when it invades into areas where it has space to move. Cases in point are the wheel well and engine block of your car. If your splash shield breaks down, one of the first thing you'll hear is a lot of splashing noise where there was none before. This is actually very noticeable as it breaks the normal "rhythm" of your car sounds that you may have acclimated to. By the time you hear these noises, it's best that you check immediately in the area where you can hear the noise-either the wheel well or the engine block. If it's a wet day, it's even better if you stop your vehicle immediately to prevent worsening the situation.
If the above symptom is paired up with slipshod performance from your engine, then the damage being done is far more serious than you initially thought. The water, by this point, might have already penetrated the engine itself-causing if to choke and sputter. Continued operation risks exacerbating the damage, so your best option is to pull over by the side of the road to assess the situation. There are, of course, other factors that go into engine difficulties but this is one of the worst ones. Any water that messes up the engine might do so permanently. It's a better idea to keep the splash shield well-maintained.