Regarding this specific behavior, PCs wouldn't be omniscient anyway.
Can't the PC simply RP, "I didn't see your notice until now and complied immediately when I did"?
And the garda RP the response IC.
While true, there's no way to prove or disprove what they've seen on a forum. It's assumed if they can reply or see the Market Tab, or whatever other announcement there is, that they have an equal chance to see whatever is coming from the Garda. They've just elected not to pay attention to it, or worse, saw it and decided to cheese an excuse. There's no real way to enforce, prove, or disprove where someone has been and what they can see. Thus, you have to just take it ICly on how lenient you're willing to be, and how genuine their reasoning is.
I.E. Outlander Bob got summoned. Outlander bob is basically always around Barovia, or out adventuring. He comes back often enough. He's probably liable to see it, there's no reason he shouldn't.
In addition, Dementlieuse Paul got summoned. He is rarely in Barovia, leaves Dementlieu to go adventuring, then returns to the Mist Camps or Port-a-Lucine daily. He might have entered Vallaki twice or so in the last month, and when he does go to Barovia, he generally goes to the Village of Barovia because that's where he grinds.
Garda Domn Domnescu sees this, and goes, 'Well, Outlander Bob is here like, three or four times every week, and I rarely see Dementlieuse Paul. I'd be willing to wager Dementlieuse Paul is just ignorant of it, and his habits support that excuse. Outlander Bob however, has little excuse not to be paying attention to the noticeboards of the municipality he most often frequents. I don't buy it.'