When to stop assuming it is settling in

The dip lifts. What is worth attention is a pattern that does not: weeks rather than days, withdrawing from people rather than being busy with them, stopping attending, changes in sleeping or eating that persist, or saying plainly that they cannot cope. The distinction is duration and direction, not how intense any single evening sounds.

If you see that, say warmly and directly that you have noticed, ask how they are actually doing, and encourage them to contact the institution's own support service. Every institution has one, they deal with exactly this constantly, and they can act locally in ways you cannot from far away. If you are worried about their immediate safety, contact the institution yourself and ask for advice.