You are likely blind to it at low levels, as am I and possibly a hefty slice of the population. That is why I say that I detect it by mouthfeel and work backwards. It was only through a tasting class by a well known local BJCP judge that I was able to determine that. Seibel and others sell a tasting off-flavor kit to assist beer judges in gaining acuity toward common faults, diacetyl being one of them.
As to your own beer - a forced diacetyl test can be run, which will help in detection:
I have had some examples of Pilsner Urquell that SCREAMED diacetyl. Some cans and draft versions were very noticeable. Some bottles were better. When I drank it in and around Prague it was much, much cleaner.
While in Prague, and Plzen (visiting the brewery), we never noticed any diacetyl. When in Frankfurt, I always went for Czech beer vs German beer.
The only off flavor noted in bottles here is an old, cardboard taste. Simply from not being a fresh beer. This trait is shared with many imports, unless you can snag a 6 pack that is only 3 months past it’s packaging date.
Depends on where you buy it. I can get fresh, well treated PU, Czechvar/Budvar here. All exhibit diacetyl. Is it possible that you have trouble picking it up? That’s not uncommon.
I had no trouble detecting it in my beer. We had a couple batches that were loaded with diacetyl. Luckily, they were fixable and turned out very good in the end.
The butter flavor in Pilsner Urquell must be very subtle. Yes, maybe below my sensory threshold.