Long-term
She left years before she left. The routines continued (dinners, weekends, the surface of a marriage) but the decision had already been made. She was waiting for the conditions to be right: the finances positioned, the kids old enough, the exit less complicated. You were living with someone who had already left.
You found out when the timing was right for her.
The game
Separation is expensive and disruptive. The costs vary significantly depending on circumstances: when the kids are younger versus older, before a promotion versus after, before certain financial decisions versus after. A spouse who has decided to leave has an incentive to wait for the moment that minimizes her costs and maximizes her position.
This is a game of asymmetric information. She knows she's leaving. You don't. While you're investing in the marriage (making joint decisions, building shared assets, staying in place for reasons that no longer exist), she's positioning for an exit.
The equilibrium
The equilibrium holds because there's nothing to observe. She's still present. The relationship looks normal from the outside and mostly from the inside. The planning is invisible. The incentive to stay quiet is strong. Disclosure would trigger exactly the disruption she's waiting to minimize.
Dominated strategies
Assuming that presence means investment is the dominated strategy. Someone can be physically and functionally in a marriage while having made a different decision privately.
The signal to watch for is a partner who has stopped making joint plans that extend far into the future, who is quietly building independent financial footing, who has reduced exposure to the shared life without an obvious reason. These are rational preparations, not necessarily signs of anything. But they're worth noticing.