Approval notices for each petition, admission records for each entry, passports covering the whole period including expired ones, and travel or expense records that corroborate departures. Employer assignment letters and payroll history help confirm which entity the employee worked for during each period.
Expect to rebuild part of the record
Records go missing over several years, particularly for short trips taken early in an assignment. Corporate travel bookings, calendar entries and expense claims can fill gaps where a stamp is unclear. Start the retrieval early, because some of it depends on systems the employee no longer has access to.
An incomplete record is not fatal, but it should be presented as incomplete rather than smoothed over. Start the retrieval with whatever depends on someone else's systems, since that is where a multi-year record most often fails. Corporate travel bookings, expense claims and calendar histories from earlier years may sit in systems the employee can no longer reach, and a former manager may be the only person who can authorise access.
Ask early and ask a named person. Where a gap cannot be closed, present it as a gap with a note of what was searched, because an incomplete record honestly described is workable and a smoothed-over one is not.