Are new ones forming all the time? Could you set an alarm and go out a few different times in the night (on different days, of course)? Is the soil piled around the hole, or thrown around? Could it be something digging for grubs?
I’ve had these holes too, in a lawn on a front garden in NW London. The slime is definitely associated with the hole. I dug one up and found a fungus like thing at the bottom. It was egg shell shaped, about a 1cm in diameter and dried out to become papery. I’d love to know what causes them. Are the slugs attracted by some sort of fungus?
It could be a type of ink cap fungus, which grow very quickly and 'melt' just as quickly. That would account for the hole which would be made as the cap pushes up before expanding. Judging by the various fungi I see with holes in, I'm pretty sure slugs love eating fungi. This 45s video is a time-lapse over 3 days:
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