The ex-First Daughter -- who's 22 years old now -- was spotted out in WeHo Friday doing a little supermarket pop-in at a Trader Joe's there ... and as you can see, she was venturing out solo for the food run, without a bodyguard in sight (more on this later).

Sasha had on headphones and was wearing a pretty relaxed 'fit -- crop top, etc. Nobody seemed to recognize her, and no one bothered her either. She was just doing her thing.

It might not seem fascinating on its face -- after all, we know Sasha's been living out here in L.A. for a quick minute now ... as she's often spotted hitting the town, especially Hollywood.

Ditto for her sister, Malia ... who's also living in L.A. now and working as a professional screenwriter. Both of the Obama girls are usually rolling solo -- without any SS protection.

If you've been wondering why that might be the case -- seeing how their mom and dad are always surrounded by government-sanctioned bodyguards, and will be for the rest of their days -- the answer lies in Secret Service policy ... which says the kiddos get cut off after 16.

This explains why Sasha and Malia are almost always on their own ... and look like ordinary citizens. At this point, in Uncle Sam's eyes -- they very much are. Also, remember ... Barack has publicly said his daughters didn't like SS trailing them when they were teens, jokingly saying they had "PTSD" from the whole experience. So, yeah -- they've outgrown it.