2. You are visible very briefly when you reapply Tonko. While unlikely, it is possible to get aggro'd by sight only mobs while doing this. Saw this happen to a RNG/NIN looking for his AF coffer in Monastic Caverns. He was quite surprised and none too happy.
i just wanted to comment on this statement, and offer a simple test to the poster.
ok, here's the deal: tonko doesn't "stack" perse. it simply over-writes the previous tonko. in my experience, tonko lasts 3 minutes. after that time you get about 4 warnings then it drops. lets say you recast tonko with 30 seconds still remaining. when you recast it, you lose those 30 seconds, but now have a fresh 3 minutes. so it never really "drops." and you CAN NOT BE DETECTED during that slight animation time when it seems that you become visible.
here's how you can prove it to yourself: go to bibiki bay, castle o, or anywhere else that has sight detecting AND tracking mobs only. gobs and yags are perfect for this. stand nose to nose w/ an IT+++ gob or yag w/ tonko up, and recast it directly in front of him. if your theory is correct, you are a dead man. luckily, this isn't the case! in fact, this simple test will eventually make you a lot of gil when you can tonko-->steal a gold coin-->hide-->tonko without moving from the mob's line of sight.
slightly off topic, i've actually noticed that my rogue's vest allows hide to last for 4:30 seconds, almost the entire time needed for steal and hide to charge. this saves me a ton of gil in shinobi-tabi, and makes gil/hour rate go up substantially assuming i'm lucky with my steals. you can also skill up evasion this way. steal, let mob take a couple whacks at you-->hide-->rest. rinse, repeat :)
gl, happy hunting :)