Posts on X are stating that the NOTAM for Biden always showed him returning home 'early', after being in Delaware for some memorial ceremony. i.e. it's not an 'early, abrupt' return. Kamala is staying in DC in case there's a tie vote on the CR and she's needed to break the tie.
Now, as to their 'not-so-secret' meeting last night - that's seems to still be a complete unknow.