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This is interesting observations indeed, but I need more information to give good answers. You write you have identified two species of the genus Brevicornu. This does not match the observation of short antennae. Do you have images? I suspect it is rather species in the genus Stigmatomeria (formerly regarded as members of Brevicornu) or Cordyla. Both these genera have shortened antenna. Stigmatomeria is known to infest truffles, but nobody knows exactly how they do it. Their antennae are not very short, but swollen and full of sensory pits. Probably they can smell their way to truffles. Cordyla have shorter antennae, and their sensory pits are in the palps. Their larvae infest mushrooms from the ground and up, and probably the female lay eggs before the sporophore emerges. My best guess based on your info is that it may be Stigmatomeria adults hatching from truffles underground. How deep was it? Which season?