I don't know if it's the browser (I use Chrome), but I have the same issue on all the forums I watch. Some of them are based on the same forum application, but only a couple. In all cases, they require me to sign in after not visiting for a while *on that computer/tablet*. Even if I was on this morning and went to a different device where I had not been on for a while, I will need to log in.

So, it seems to be the browser (not sure if all work this way). With Chrome there is no expiry date for the password cookies (as far as I can tell).

I have not tried other browser.