please don't shoot me for saying this but are you sure they are not mushroom slices? I only dare to say this because some months a go I passed some horrible looking things that looked like large black slugs......it turned out they were mushrooms I had not chewed very well the day before....and they looked gross! since then I have noticed that any mushroom, button, sliced etc......is not digested at all.
I have made so many mistakes over food items that have passed thru relatively undigested that I always err on the side of caution. if there is even a 10% chance it is a bit of veg or something...then I discard it as unreliable evidence!
Of course that does not mean you are as daft as I am....