As an additional test, try leaving some water on your samples without wiping it up for a while.
I have compac Quartz (Luna) and I'm driven bananas by babysitting it. I have it around the sink and on a large island.
It's never stained but if I dont wipe up water or liquids straight away then I get a sort of shadow mark. Also have to be careful not to use any cleaning products on eg if sprayed on, you'll see the spray marks.
You can only see these marks at certain times depending on how the light falls. We have high, large veluxes and it's the light that comes through these that makes the marks visible. You can't see them in artificial light unless you squat down. There is also tonnes of glass in our kitchen.
The area around the sink must be hellish but I can't see the marks due to the way the light falls there, it's slightly in shadow.
These marks do eventually go but it takes months.
I was wandering around Lakeland one day to see if there was something I could do about these wretched shadow/evaporation marks.
An assistant approached me and asked if she could help. I started to say I had quartz work tops and got no further before she interjected and said, oh are you getting shadow evaporation marks? She said they'd had lots of customers come in about the same thing. So I bought something called Worktop Wonder which masks the marks.
I'd be very careful what cleaners you use. You should only use cleaners that are ph neutral and Bar Keepers Friend isn't.
That was long but cathartic. 