I'm trying to understand... yesterday at 15:59:59, GXY bid at...

I'm trying to understand... yesterday at 15:59:59, GXY bid at 200, offer 201. Closed at 201. I run a "shenanigans" monitor that looks at the bid/offer at 15:59:59 and then compares with the close price (almost always a closing auction match price). Then measure Close less the mid price, as measured by the number of "average spreads". In GXY, most of the time it is near a 1 cent spread. Some high priced stocks, like a MQG for example, trade on a wider spread than the minimum tick size. A reasonable comparison might be something like OZL... it was 1020/1021 and closed at 1019. If it was GXY then "j'accuse!", but the movement isn't that unusual. A move of 3, 4 or more spreads is quite rare and worthy of looking into.

In GXY's case yesterday, it didn't flag as unusual - fun fact: you get lots of stocks with flags on the last trading day of the month, even on large cap stocks, which I attribute to either window-dressing (very not-allowed) or month-end MOC rebalancing flow (quite routine).

I'll have a look at today's match and, importantly, see if it "looks like" other stocks. Analysing it on its own really can't confirm or rule out anything.