Hard to believe the hostas didn’t come up, they are usually very tough plants. Things to consider would be de icing salt water runoff, weed and feed accident after fertilizing a lawn, lawn company mistake, vole or gopher  damage could be a factor but as you pointed out one would expect to see the tunnels, and if the edge of the patio is shady they might not be up yet. Flooding seems an unlikely cause since hostas survive drought and standing water for short periods. I am sorry I only have speculations to offer.