updated 04/14/2014 AT 11:00 AM ET
•originally published 04/14/2014 AT 11:30 AM ET
Friends of the couple “wouldn’t be surprised” by a proposal, says the insider. “But she’s not engaged – yet.”
The two took their romance public in January at Penn’s third annual Help Haiti Home gala after appearing to be more than just friends during a holiday trip to Penn’s beachfront home in Oahu, Hawaii, where the pair celebrated the New Year together.
The Oscar winners then spent Valentine’s Day weekend together in Cabo San Lucas.
“They were inseparable,” an eyewitness told PEOPLE of their Mexican vacation. “They held hands everywhere they went.”
Penn, 53, is also planning to direct Theron, 38, in the love story, The Last Face, costarring Javier Bardem, which will likely shoot in Africa.
Reps for the couple have not commented.