DNO Lucy box heaters

I read on a Facebook group that deals with old plugs and other equipment that some Lucy boxes have small heaters built into them to keep the damp out and keep the mice warm. Is this a common arrangement? I'm just curiouse because it seems like the DNO would need to check these fairly often also surely the internal fuse gear would generate a fair bit of warmth especially on a winters nite. Ide be interested to know what our regular DNO engineers here think