Why your options “moved” after 3:30 — marking, not trading
You close the terminal at 3:30 slightly green. At 6pm the app shows the same position deep red. Nothing traded in between — so what happened?
Marks, not trades
During the day your broker displays the last traded price. After the close, that is replaced by the exchange's official settlement price — a weighted average of late-session trades, or, for strikes that barely traded, a theoretical value computed from the index close. An illiquid strike can carry a stale price all afternoon and then be corrected in one step. It looks like a crash. It is accounting.
The closing auction's role
The auction sets the index close, and the index close feeds those theoretical option valuations. So the post-close jump in your P&L is downstream of the auction print — which is itself a different beast than it used to be (see our closing-auction piece).
What to actually do
Nothing, until 9:15. The mark is not a price anyone will trade with you at. Check the live bid-ask at the open before making any decision — that is the price that exists. A marking correction says nothing about tomorrow, and reacting to it at 8pm is reacting to bookkeeping.
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This article describes market mechanics for educational purposes. Nothing here is investment advice, a recommendation, or a forecast — conditions, never calls.