Update. Last Saturday 'er indoors gets a call from Tesco customer services. Not a call centre but the desk at our local store. (#2 son is known to them because every time he's in with Mum he collects all the scan'n'shop handsets and returns them to the racks whilst they smile indulgently.)
'Hi. Is this Mum? Have you lost a mobile belonging to a tween? Sorry but we had to turn it on to see if we could identify who it belongs to.'
Kind of explains why we couldn't find it despite turning the house upside down. Nothing nefarious, I was fearing an astronomical bill but the only calls were the umpteen we'd made trying to find it. Any road, #2 son is reunited with his phone and all is well with the world - provided you squint a bit...