How long can liquid be stored in your computer?

I have a macbook, and somehow I woke up and the screen was cracked I dont remember cracking it but it was on my couch, and when i was about to go to sleep I closed it, but the screen was black when I did so I didnt see the damage until the morning after. Anyways I took my laptop to get repaired at the Apple Store, and now they are telling me that there is liquid in my computer, and want to charge even more money to get it repaired, but it was working absolutley fine before I sent it to be repaired, and I dont remember ever spilling anything on it. But even if a couple of drops were spilled on it, wouldn't it have dried , and the laptop just be damaged by now? How bad could the liquid spill be if it was funtional before the cracked screen? Mind you, it has been in Apple repair for over a week, and now they are finding liquid. I do not know much about computers but it sounds like a ton of bullshit to me, and its a lot of money. So to sum up my question... Is it possible that someone there could have spilled liquid on my laptop and are expecting me to pay for it? Or could they be lying? Or am I just being cheap and paranoid? Please give me some honest answers here.