I also think the updater checks more often than just on reboot for updates. I think killing it and reopening should do the trick either.
I don’t know if it’s there on FPOOS, but on FPOS (with GMS), there is a “developer mode”, which can be activated by tapping the version (“Fairphone OS 1.7.1” in my case) several times. When you enter http://ota.fairphone.com/, it sends a request every time you’re hitting OK.
