I use Firefox built-in password manager with master password. Works on Windows, Linux, Android. I haven't tested but should also work on IOS and OSX. It's opensource, does not store unencrypted passwords in the cloud, uses OS popup for master password prompt, and prompts only during browser startup, so it'd be very suspicious if opening a page would show the prompt. Also knowing master password is not enough to compromise it remotely - you'd also need Firefox account password which shoud be different and very complicated.

At work I've developed a system for exactly this in 2004. It was based on procmail. It is obvious.