I actually saw this happen at a funeral mass, of all things.
I don’t know how they caught it, but all of a sudden the altar serves etc. hustled over to a woman who was trying to slip out the side door with a eucharist.
At a Novus Ordo Mass one extraordinary minister of Holy Communion saw a gentleman she knew slip the host into his Bible rather than consume it.
Since she knew him, she went to his home and asked for the host, which she then consumed.
Although I try not to let it distract me, as an EME I do find myself watching whether people do consume the Eucharist. I do think I would stop someone if I saw he or she trying to take it back to the pew or sneak out with it.