Hi Maria, nice to see you here!!!
I wouldn't call it weird, necessarily, but I would call this kind of behavior disrespectful -- particularly if the guest never bothered to send you a message to tell you what had happened (YOU had to take the initiative to contact the guest, after finding sheets missing) and particularly if the guest didn't offer to pay you for the sheets. It does happen that people can have "accidents", but it's just normal decent behavior to let someone know if you have messed up their property via an "accident." Not telling them at all, and just throwing their property out, is NOT okay!
Precisely to avoid things like chocolate milk all over sheets, I state explicitly in my house rules that guests cannot consume food in bed.
I've noticed that hosts are split on whether they bill the guests for damages like these. Half the hosts seem to take the approach this is normal wear and tear, and don't bill the guest. I'm of the opposite camp, I take the approach that guests are not entitled to expect that their reservation comes with a free set of sheets and a free set of towels, so if they damage any of those, I will bill them for it. Unfortunately I dont' always notice the damage in time. Sometimes it's only when I am folding the laundry 5 days later that I notice a stain on the linens, which makes them now unusable for guests.
I can understand guests feeling embarassed by massive stains, but leaving one or more $20's in the room to cover costs would be appreciated, or just the guest fessing up to it and expressing willingness to be billed thru the security deposit.