The question remains why did he take it?

In reply to a post by John_ON:

Pocketed my ADSLNation faceplate too!

If this style
http://www.tandyonline.co.uk/nte-2005-master-facepla...
then perfectly reasonable to have assumed it was a BT installed one.

If this style
http://www.tandyonline.co.uk/xte-2005-master-facepla...
then should not have taken it, as that style has never been used by BT. Poster is at liberty to report it to ISP who can pass it on to Openreach.

The OP already mentioned his "ADSLNation XTE Faceplate"

Cannot keep track of everything in a day

He likely took it to discard it in the same way other obsolete equipment is discarded.

The BT Openreach engineer who installed my FTTC took the modified faceplate filter I bought from Clarity, and I expect a perfectly good filter landed up in the scrap bin. I could have stopped him by pointing out it was a modified version that was clearly my property, but I couldn't be bothered. He'd been neat, tidy and courteous. He let me take pictures of his JDSU screen showing the line test results, though he did insist on me covering up the logos on the unit itself first. He waited whilst I reconfigured my router and assured him all was well with the connection. Mine was the first FTTC installation he'd done that didn't involve a BT Home Hub or similar router - in my case the modem connects to a dedicated VLAN on my network and the router is a rack mount server running pfSense in another building, connected to this one via a couple of Gigabit fibres.

I finished up with a working installation. The engineer went by the book, scrapping the existing NTE5, replacing it with a new IDC NTE5 (sometimes called a CTE5) and interstitial filter. He moved my extensions to the faceplate of the new NTE5 and insisted on testing them. I have nothing to complain about.

I probably should have a spare filter, but the interstitial filters aren't expensive. When I get round to it, I'll buy one for my spares box.

My engineer handed me the old Openreach faceplate saying it was mine.

I had mine installed nearly 2 years ago and I still have my ADSL Nation faceplate fitted, all he did was open it up to check that the wiring was correct, he may have removed the bell wire if it had been connected in the first place. Somewhere I still have my original BT filtered faceplate from when I had Home Highway replaced by ADSL.