No need to cook the shells to 'disguise' them... but best to not mix into feed, keep in a separate container for them to eat as needed. I rinse, dry, save, then crush to mix in with oyster shell in this:
Ditto Dat^^^..some animal protein is good too.
Maybe might have been 'thicker than egg white' because it was cold and started to freeze? Or was just the thicker part of the albumen.
Could have been a thinner shell and got kicked out of nest and broke, hard to tell if they've already eaten the shell when you in find the 'wet spot', BTDT. Took me 2 weeks of finding messes before catching one headed out of the nest with shell in beak, had to chase her down to find a very thin shell-thus easily broken and gleefully consumed.
Do your nests have 3-4" front on them to reduce the chance of eggs getting kicked out? New layers are often rather rambunctious about 'fixing' the nest before laying. Curiosity pecking
can become a habit, good to gather often if finding wet spots as well as adding fake eggs to nests.