Hm... I actually agree; the messages regarding summons aren't particularly clear, IIRC: "The boggart mumbles some strange words." and a bunch of things pop on the screen. I'm pretty sure demon summons aren't any more clear -- they simply gesture.
Maybe being more explicit would help: "The boggart mumbles some strange words. Some monsters appear!" Or, if it's invisible, "Something mumbles some strange words. Some monsters appear!" Maybe "are summoned" is better than "appear" in this context.
Letting people know that monsters have appeared via means other than normal monster generation seems like a good idea. Might be a good idea to let people turn this off, to keep summonspam fights from being even less spammy...
@JasonMel: Kill summoners first
I am not a very good player. My mouth is a foul pit of LIES. KNOW THIS.