Quoting myself from SeaFlags:
"At least as early as the first decades of the 20th century, a series of special signal flags had been adopted for use by yachts to convey certain messages. Some of these are still in use, at least in the old line yacht clubs aboard large sailboats [as well as other large vessels]. The solid blue "owner absent" flag is flown from the starboard main spreader to prevent visitors from having to pull a rowboat across the harbor in order to find out that only the hired crew is aboard to receive them. [Remember that these were invented in the days before radios and cellular telephones.]
image by Jose C. Alegria
The same flag with a white diagonal stripe flown at the starboard spreader indicates that, although the owner is not aboard, a guest is. Just in case you are coming across to visit the guest instead of the owner."
In other words, the guest flag is not flown whenever a guest is aboard but only if the guest is aboard and the owner is not.
This is jocularly known as "the dinner napkin." At least that's what the books say. I'm not a yachtsman.
image by Peter Hans van den Muijzenberg
The crew's meal is signaled by a red pennant.
Coffeepot: coffee is being served.
Foaming stein: beer is being served.
Martini glass: cocktails are being served.
Ball and chain: wife aboard.
Battle-axe descending: mother-in-law aboard.
Witch on broomstick: wife has gone ashore.
Witches on broomsticks: the ladies have gone ashore.
Snuggle-bunnies: we are in bed; please do not disturb.