Collect the current I-94 and admission stamp, the original support letter describing the assignment, and written confirmation from the employer of the actual end date. If a new role is in prospect, its offer and duty description belong in the same file.
Build one file covering how the assignment started and how it ended
The original letter shows what activity the admission was granted for; the termination or completion notice shows when that activity stopped. Together they let counsel assess the gap precisely. Keep travel history, prior extension notices, and any correspondence about the shortened timeline.
If a departure follows, retain boarding passes and entry records for the destination as evidence of when the person actually left. Two records tend to decide disputes in this file. The support letter written at the outset governs what the admission was granted for; a later notice governs when that activity stopped.
Anything else, whether an internal calendar, a project tracker or a colleague's message, is context rather than proof. Where the employer's systems produce conflicting dates, request a single signed statement listing each date and saying plainly what it represents. What goes wrong is usually the order of collection: people gather the new offer first and the closing records last, then discover months later that nobody kept the termination notice.
Assemble the file in the week the news arrives, while human resources still has the matter open, and keep a copy outside employer systems whose access may be withdrawn.