Thanks for the suggestions.
you could have a big pulsating heart in the bubble.
I like this idea... I might try it out when I get the chance.
Maybe the classic "circling stars?" Or a lump on the head?
This is interesting, and I might start looking into it. The thing I was going for with the speech bubble was trying to create a format for what states the guards were in. The '...' above that one guy's head is supposed to denote that he hasn't noticed the player and is just going along his regular patrol. There are other symbols for different states of alarm. I thought I'd give bodies the same symbol, because you'd need to be able to tell whether they were dead, or they could be revived.
Or when you use heat vision he has heat in his body?
The Heat vision suggestion reminds me a whole lot of the airport mission from Pandora Tomorrow, where you had to kill the workers with the raised body temperatures. It won't work for me however, because I'm not using heat vision :/