The barking is a weird one @VanGoSunflowers . Is he seeing a cat? If it's a dark coloured cat he might just be seeing its eyes and getting spooked. Or is he just hearing something that you aren't registering? The collie next door to us barks every time it hears an owl. Which is not so great at 3am 🙄
Normally I'd say adolescent male dog with female owner, be wary of him becoming over protective, but this doesn't sound like that. And anyway, I credit you with having enough nous to recognise that sort of possessive behaviour and nip it in the bud.
It's a strange one with the groomer or @SpanielsGalore 's decorator, because, as friendly as Brie is, I wouldn't be able to tell someone to come in through the back gate if I wasn't here. People who meet our dog(s) always think they're big softies that love everybody, and yes, if we are there, they greet people nicely. They don't bark at the postie or the bin people, because that's normal everyday stuff, but if a stranger was lurking they'd let us know. Whether they'd let a neighbour in would depend how well they knew the person; the ones that back onto us, maybe with Brie, but with our previous dog Algy, probably not. My cousins could definitely come in without us being here and they're our emergency back-up. That natural wariness and the ability to make a distinction between friend and foe is, I suppose, the biggest difference between a guarding breed and a dog that just barks at the doorbell.
Either way, I hope you can get to the bottom of what is spooking Pablo before it becomes too much of a habit.