Well, the hose off the coolant tank dump shows "when" it's losing coolant. (On the turbo cars, it's when you make boost, it just blows coolant out like someone turned on the garden hose. Then it mostly stops when your not boosting.)
If your missing the first 6" of your undercover, that might have something to do with this, but only a minimal part. It's just going to dump air that come in through the grille out the botterm v/s being pulled through the radiator. Either get a complete one that goes from the bumper cover to the oil pan, or at the very minimum, from the bumper cover to the lower tie bar under the radiator.
I'm guessing your not running a shroud anymore? That hurts fan performance too as air is just pushed off the ends of the fan blades, and not really moved the way you want it to. (Think air planes. Prop planes are not as effective as jets due to the shroud around the jet engines.)
I think it was noted, but by chance did your E-fan get wired wrong so it's pushing air v/s pulling it? (Assuming it's been mounted on the engine side of the radiator.) With it setup like this, the car would idle fine, and might even be fine around town at slow speeds and in traffic, but at highway speeds, the fan is trying to move air "forward" against the incoming air from the grille, and it would cause a problem.
Your mechanic should have a gauge/tool that replaces the radiator cap with a pressure gauge and pump/shrader vavle. (To pressureize the system) This is designed to check for leaks. I think if you leave it on the car while running, it would show any pressure spikes, or vac conditions as the coolent is sucked out. (Blown HG for example.) When running, the pressure gauge should come up as the system heats and expands, but it should not bounce too much.
Last, if they have not taken the pump off and inspected the pump vanes, they don't know what kind of condition they are in. Rusted vanes, or missing vanes will not move the coolant properly, and might be causing your problem.
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