If you never quarantined, then ich has probably been in your tank for years.
From what I've seen with ich management on here, ich will basically remain managed and under control (seemingly nonexistent in the tank) as long the fish are in excellent health (and as long as the outbreak doesn't get too bad for the tank - not necessarily for the individual fish, but for the tank as a whole). As soon as a fish gets stressed/weakened, however, there will be an outbreak like you just had with your tang.
To say it another way, your tang isn't really cured, it's just healthy enough to hold the parasites at bay at the moment. Your other fish likely have ich too and are just asymptomatic at the moment like your tang. So, if something stresses/weakens them, you'll have another outbreak that seemingly appears from nowhere. To prevent another outbreak, you have to keep the fish in great health - low stress, good food (LRS and macroalgae are great; some of Hikari's stuff is really good too - like their Mega Marine line - but some of their other marine-oriented food options aren't as good from what I recall), good water quality, etc. are key.
This is why I typically suggest for people who are considering not quarantining to look at some of Paul B's ich management stuff, as he talks about good care being necessary for good fish health too. (This is also why I typically suggest that newcomers to the hobby buy pre-quarantined fish if they don't want to do a full, medicated QT - fish that don't have diseases/parasites should be a lot harder to accidentally kill, as they won't have a potentially lethal disease outbreak every time the aquarist makes a mistake while learning the ropes).
