Lotus Milestone is mentioned in a scan in Quarantine Monitor in the Phendrana Drifts in Metroid Prime. It is possible it could be a date, event or a deadline that the Space Pirates have. The scan indicates that Thardus could be installed as a patrol guard in the Phazon Mines by Lotus Milestone.
"The taming of Thardus continues. Barring unforeseen attacks, it can be installed on Phazon mine patrol by Lotus Milestone."