Actually I had a similar experience with a 5 dollar bill (these are not usually forged). The storekeeper said he couldn’t accept it because it didn’t “pass” his device but he wasn’t sure it’s a fake either. So he returned it. I think his machine went by color. To me it just looked washed out and I was kind of offended.
I didn’t bother checking with a bank, but I don’t think they can confiscate it based on flimsy evidence. This was many years ago, I don’t know what testing methods stores use today.