Yes, or if your fast, cheap new metro ethernet 10GbE circuit between two buildings has been provisioned by your ISP as some form of transport (handoff into a WDM system, EoMPLS tunnel), etc, it's not difficult for them to 'mirror' your port. It could be pre-tapped before it was ever turned up for customer service.

Or the splitter could be installed in your riser cable at one end, before you ever started moving packets across it.

Or a myriad of other things.