Welcome to the community!
It's possible that your office is located outside of -- or just on the edge of -- Fido/Rogers coverage, but within the coverage of another Canadian provider. As such your phone is likely connecting to their network using extended coverage (Fido-EXT). Technically, devices using extended coverage are roaming on the other networks. There is no additional charge for using extended coverage. However, you should note that its use is meant to only be on a temporary basis. If the majority of your monthly usage occurs within Extended Coverage areas, Fido may restrict or limit your access to Extended Coverage on an ongoing basis.
Hope this helps