I don't believe Canon does 'stealth' updates. Heck, when they updated the 18-55 and 55-250 kit lenses with a couple of cosmetic changes to make production a tiny bit cheaper, they announced them as MkII versions. They do correct problems without an update, for example the 'rays of light' flare on the 24-105L that was fixed with the UT11xx and following production runs (and older lenses could be sent in).
Of course, the longer a lens has been in service, the more likely something is wearing out, or has worn out, or internal damage (e.g., decentering) has occurred.
In 2010, I bought a 200/2.8L II with a UL date code (1997, second year for that lens and six years before the 17-40L was even released). It was optically and mechanically excellent (and a great deal at $450!). I sold it only after getting the 70-200/2.8L IS II.
The key is to test the lens thoroughly.