Honchos at Francoeur French Bulldogs, a breeder in California, tell TMZ ... stealing Frenchies has recently become a thing -- it's happening in Los Angeles, San Francisco and Phoenix -- and they're advising French bulldog owners not to walk their dogs at night.

TMZ broke the story ... dognappers abducted 2 of Lady Gaga's Frenchies at gunpoint this week in Hollywood, stealing them from her dog walker as he was taking them down the street around 10 PM.

Folks at French Bulldog Puppies Los Angeles say they're telling their celebrity clients to be extra careful with the dogs, because thieves know Frenchies are in demand and expensive. We're told puppies can be worth up to $20,000 and older dogs could still fetch $2,500.

Lucky for Gaga, her dogs were returned unharmed Friday night when a woman dropped them off at a LAPD station. Remember ... Gaga offered a $500,000 reward for her dogs' return, no questions asked.

French Bulldog Puppies has a message ... the most important precaution folks can take is microchipping their Frenchies so they can be identified if they're taken to a vet.

Frenchies are small, in-demand and worth a lot of money, which puts a target on their back ... and the owner of French Bulldogs LA says the problem's compounded by the fact the dogs are so friendly they would hop into a stranger's car with no problem.