That, more than anything else, captures Mirrorton's attention. As much as he'd like to think that Mettaton tells him everything that happens, he knows he's lying. He knows he's holding back because he still won't tell him everything about his friends. He'll only give him summaries of what happens in the events. But the tides are changing. He can feel it. He just needs to keep pushing. And get rid of pesky roadblocks like this skeleton.

It doesn't matter. I always find out. I always know everything, in the end.

If he knew about Mettaton kissing Sans, there is no way the mirror could hide his revulsion of the skeleton in any capacity. And Mettaton would not be allowed to leave his room for a week as punishment.

Mirrorton grinds his teeth, glaring daggers at the Monster before him.

Then you will face the rightful consequences that you deserve. A punishment befitting your intrusive nature and foolish assumptions.

Sans's eyelights switch off and Mirrorton's glare increases in intensity.

The only waste here is you, Sans. Never forget that. But I will take my leave. I have better things to do than to speak to you now that I have said my piece.