The supermodel shared this hilarious anecdote with Alex Cooper of 'Call Her Daddy' this week -- explaining that her comfortability with being in the nude most certainly extends to how she lives within the privacy of her home ... where she's apparently topless quite often.

HK explains ... "I mean, I try to be like that with my kids. Obviously, when you know, no one is there. I’m in my garden also, you know, I’m topless," Klum says. "And they will come, like, 'You know, my friend is coming over, mom, put your top on.' And I’m like, 'When have I ever not had a top on when your friend was coming over? Of course I will.'"

Even with her guarantee, though, Heidi says her kids always double-check to make sure she is, in fact, clothed when their buddies roll through ... and Mama Bear says she complies.

Funny enough, Cooper actually called Heidi's oldest daughter -- 19-year-old Leni -- during this chat with her mom ... and the teen said, yeah ... Heidi goes topless a lot, noting it's been pretty embarrassing over the years. With that said, she says she's gotten used to it.

It might not be that big a deal around her girlfriends, but you gotta figure any boys that are being brought to the Klum household have gotten giddy at the prospect of a visit.