I'll point out that this is why many people don't like random people to be drafting them, because they don't want to feel responsible for having someone behind them. When this comes up here on BF all the time, the common thing people say is that it doesn't affect the person in front so they shouldn't care. But as you can see, it clearly does affect them because now [some of] you guys are saying that it's "bad etiquette" to not signal to some random stranger sucking your wheel.
The bottom line, if you aren't in a group with someone, and you are following close enough that it is a problem when the person in front doesn't point something out, then you screwed up. The etiquette fault lies with you for getting too close in the first place.