If an existing monitor/mirror feature of the platform or device is used, it’s not exactly intrusive or as easily remotely detected. But an hardware implant non the less. Assuming you don’t have to actually configure the active gear, to send specific light/wavelengths to the monitor port

I semantics. If the actual cable is getting “tapped” with a splitter (previously linked) of some form that didn’t exist in the path, it would be a a clear hardware implant

An example of a passive splitter cable, that could be connected upstream of a fibre service to monitor the light in one direction with s fair degree of opticoa loss - www.ebay.com.au/itm/35228174...

Yes (ish). If it’s a passive mux a splitter would be required unless the mux has a monitor (mon) port inbuilt. Mon ports are commonly used for testing using spectrum analysers. If it’s an active mux (WSS style RODAM), individual waves or the whole incoming spectrum may be output for monitoring