Bummer.
I'd put him on his rug and give a full body massage... see if you can find any sensitive spots. Could be his tummy, too, if that's been upset lately.
Make sure you're comfortable touching him everywhere... he might react but not grab for you, where for the others he's grabbing as he's reacting...
You might also want to introduce him to a 'happy' muzzle... so you can maybe keep him closer to you safely when there's action about the house. Make wearing the muzzle a pleasant non-event, so neither one of you feels bad when he has it on. Let the kids know this is just his safety mask, that it's not a punishment for him. If you can find one that's fun to look at, versus a scary wire cage, that might help others view it as less imposing, too.
Another idea - put up a webcam to capture the area most likely to have 'events'... then maybe you can review the video and see if there are any signs he's giving off that can help point to the source of the issue.
You say he 'grabbed' hands in both cases... but released when asked (that's a GOOD thing). It doesn't sound, the way you've described it, like an aggressive act, more a reaction to something he was trying to stop?
Does he have a 'hard' mouth when taking treats, too? You could work on his bite a little bit... make sure he always takes treats gently, go back to 'ouch' and 'yipe' when he grabs anything too hard... see if you can soften up the 'grab'?
Seems weird that he'd walk away, then come back for a grab... not sure how that fits in doggie vocabulary...