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.