Chara can hear Grune and Mettaton- especially Mettaton- through the window, but it's muffled and they only get that there's some sort of argument going on. They don't pay attention to the exact words, just focus on creeping steadily further and further away from them.

Have to keep going... Have to keep going...

They're a bit surprised that the window hasn't been thrown open yet, but at least it gives them a chance to get away.

(It's going to be okay- For somebody, maybe, but if Chara knew Grune's thoughts they'd only be able to laugh.)