Comment 1 for bug 1658027

This does not happen only with apt-get update. A new temporary file is created by (almost) every APT command if at least one of the main cache files is outdated. It is then renamed to the final file. If APT exits before the rename, the temporary filename might stay around until you run clean. Given that this only started 6 months ago, something must be trying to read information from the apt cache.

Given that we are talking about a 4 year old release that has its end of life in 3 months, I'm not going to spend any time investigating this. There have been a ton of changes in 4 years, and I do remember some cleanup in that area, and there have been no other reports about that AFAIK with recent releases.

That said, if you can find the cause and reproduce this in 16.04, we might be able to do something about it. But it's of course difficult: There will always be some temporary files remaining if APT crashes in the wrong moment. If you run a command and interrupt it with Ctrl+C, there will likely be a file laying around, and there is really nothing we can do about that.