Except Plagueis was pretty much invisible as well. Maybe not TRUE invisibility, but he was able to sit around on Coruscant without even Master Yoda knowing he was there. He would have to literally walk into the Jedi Temple and say "Hey I'm a Sith Lord here to kill you!"
If Yoda and the entire galaxy couldn't see him, what makes you think G0-T0 can?
Plagueis isn't really a hands-on kinda guy. If he has the tools to take G0-T0 out, he will use them.
I never said GO-TO would find him. That's another subject altogether. Both are VERY hard to find, it seems like.
I was merely stating how I think the battle would go down IF Plagueis and GO-TO did go 1v1. Actually, GO-TO may even seek Plagueis out, if he recognizes the extreme advantage he has over the Sith. But first, before any kinda of offensive can be made, they have to find each other.
Plagueis is somehow able to find GO-TO's ship, and gets safely through. The sun guard (from what I read on wookiepedia) would not be with him, as they served as guards on the moon Sojourn. Unless Plagueis specifically asked for the guards to come with him, they would not.
Anyway, Plagueis is inside of GO-TO's ship, and believes he has captured GO-TO. All GO-TO needs is for multiple assassin's to attacked Plagueis at the same time with some type of paralyzing poison. Plagueis' Heart Stun wouldn't be able to overcome that, and even if he managed to kill the assassin's afterward, GO-TO could've escaped, or gotten in for the kill.
This is another valid senario, but even with his lack of awareness, I doubt Plaguies would fall into a trap.... but then again, you never know. I think poison, as it was the first time, will be the Dark Lord's downfall. GO-TO and his group of assassins are no strangers to toxins, so it is very possible GO-TO will just poison the alien, the air, or the food/drink.... if he can find Plagueis.