Ive had a look for patches that might be behind this and ive also looked at the content headers supplied by PM. I dont see any expiry header and I don't find any applied patches that this could have come from. Of the last 50 patches all but a few were by me, and none of them have to do with headers fwict. Certainly non of mine do.

So absent further info i'd have to say this probably local to you.