Each of my 4 tires are missing 1 of the 5 lug nuts. How dangerous is this?

I never paid attention or realized until I thought I had a slow leak in my tire and a mechanic told me he can't take the tire off without the lug nut I was missing. He told me they were potentially in my trunk with my spare, in the center counsel or perhaps in my glove box. He recommended i go to the Honda dealership to get the lug nut so he could then check for the slow leak. It's been a few weeks since this but I'm super confused. Why are all four tires missing one lug nut? My car is driving around normally. Does anyone understand why all four tires are missing one? How urgent is it to replace them? It it expensive to have them put on. I know lug nuts themselves are not expensive

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