If its a lecture hall, Leaving may make them VERY noticable and force others to move to let them through.

Imagine having a final thesis presentation only for one of the facualty leave mid presentation without a word.

If someone I didn't know had left, I wouldn't have minded. Even someone I did know, in fact, although I would have asked what happened afterwards and if everything was okay.

People have all sorts of emergencies.

I would have minded if the person leaving said anything though, unless I really needed to be informed of something. Better to leave in silence without disrupting the presentation. It is stressful enough as is.