Congress has a few different ways of approving legislation, one of which is just asking if legislation can be approved by "unanimous consent" where they just call for the yeas and nays and then typically say the yeas have it. To approve a call for unanimous consent the house minority and majority leadership have to approve the vote. The house minority leader (Kevin McCarthy R-CA) was not present because it was a pro forma session which means "for the form" or "for the formality" (typically they don't actually do anything during these sessions). Of course late yesterday evening McCarthy said he wanted to change some of the foreign aid spending in the omnibus bill so it's possible Pelosi sundowned before he said that and when they reconvened this morning she was like "where's Kevin?"

This was the strategy all along though. Approve minuscule amount of relief, receive massive backlash, Trump gets to look good calling for more relief, Republicans get to block all relief, and hold the government hostage into the start of Biden's term with stopgap spending.

Edit: My bad that's a voice vote, there is no vote by unanimous consent. They just ask if there is any objection.