With release of iOS/iPadOS15 last year, Apple’s Find My services are now not entirely reliant upon devices themselves having a direct network connection. While a WiFi Only iPad has no GNSS (GPS) positioning capabilities with which to derive its location - and is entirely reliant upon derivation of an approximate location based upon geo-location of its public IP Address - Apple now uses its network of millions of iPhone’s to detect beacon transmissions from Apple devices - and AirTag.
Assuming iOS/iPadOS15.x has been installed on your iPad, devices without WiFi or Cellular network connection can still be detected by other “passing” iPhones that are in proximity. With your station lockers being in an accessible public location, they are very likely to be visited by many people that are themselves carrying iPhones…
As a background system process, fortuitous device detections are anonymously communicated back to Apple’s back-end servers - along with the location of the “detecting” device. As such, while devices remain powered, they can often be located via the Find My service.