B737-900 Technician reported communication issues and the failure to follow procedures led to maintenance not being properly performed for the aircraft. The tire pressure was well below the limit; and the tires should have been changed during the tire pressure check.

Narrative

Approximately 10 minutes before scheduled departure after an overnight stay; ramp personnel called for Maintenance to look at a tire that appeared flat. A coworker found pressure at 60 PSI; well below [the] limit of 250 PSI. We changed the tire; but per B737 Aircraft Maintenance manual; we thought the adjacent tire was okay to remain in service; thinking the aircraft had not moved since the last pressure check. We were just starting our shift (days); and had not worked the plane that previous shift (midnights). Later; while discussing this with the other crew; we learned no maintenance had been scheduled on that plane. This is a VERY rare situation; as overnight aircraft almost always receive a work package that includes tire pressure checks. We discovered this plane had a work package completed in another station immediately prior to coming to our station; making up a last-minute equipment swap. Because of this; no work was scheduled overnight at our station. Had we known the aircraft had moved since the last tire pressure check; we would have changed both tires. After learning this; the aircraft returned to our station for another normal overnight check and we changed the adjacent tire as required.