Nice find !
This happens to me quite a bit.
I have flies stashed all over the place (especially in my car) in all kinds of different boxes. I make fly boxes out of old eyeglass cases, Tupper ware, Altoid tins whatever I see usable for a fly box. I have specialty boxes for all different species of fish. Striper boxes, Corbina, general surf fishing, Barracuda, Carp, Trout (separated into dry /streamer/ nymphs), Steelhead, Tuna, Calico Bass, LMB and then to top it off I have duplicates of certain boxes I use to fish with on my paddle board.
Another place is the inside brim of my baseball cap. I stash flies in the sweat band when I don't feel like carrying a box and forget about them.
So yeah this happens to me a lot