The occasions on which I go to a supermarket alone are rare. They occur only when we run out of one of two things unexpectedly.

The occasions on which I accompany my wife to a supermarket are even more rare. When she's along, even a stop for "just a couple of things" can run on for $40 or more, with frequent delays as she points out how much each item has gone up since last week. I try to avoid these excursions whenever possible.

The last time we stopped off for a few things, I wheeled out a cart loaded with almost $50 worth of food and said, "Stay here. I'll bring the car around."

"I'm coming with you," she said. "I'm worn out from walking up and down those aisles for an hour. I've got to sit down."

As we walked to the car together, I said to her, "After all the money you spent and the work you did in picking out the things on your list, isn't it risky to let that stuff sit there while we both walk away from it?"

She gave me one of those palm-out waves that a woman uses to shoo away an insect or to inform a husband that he doesn't know what he's talking about. "There's a bag boy stationed there, or loading boy, or whatever they call him," she pointed out. "He keeps an eye on your groceries for you. How do you think I get the groceries loaded into the car when you're not along? There'a a number on the shopping cart and the boy keeps an eye on it until you drive up with the proper claim number."

"It appeared to me," I said, "that the boy was keeping an eye on the girls, not the carts." But I knew Ihad lost the debate. When we drove up, she presented her claim number and the boy loaded the bags into our car.

I had almost forgotten the incident when a letter arrived from James Black of Bethesda. He wrote:

"After surrendering all that money to the checkout clerk, wheeling your groceries to the retainer fence, and leaving your expensive purchases unguarded for a few minutes while you bring your car to the pickup area, did you ever get the feeling that you'll never see your groceries again?

"Well, it happened to me. Either someone stole them or they were taken accidentally.

"Fortunately, I remembered the exact amount of cash I paid. And fortunately the clerk remembered me, and fortunately I contacted her just a few seconds before she went off duty."

The store manager said there was a good chance that somebody had picked up the wrong groceries accidentally and would soon return with them or call up.

When time passed without any such development, the manager told Black to go ahead and "re-shop" his list, on the house. There would be no charge for the amount verified by the checkout clerk.

Black is grateful, and he has some advice for supermarket shoppers.

The best protection, obviously, is to shop with a companion. One stays with the groceries and the other goes after the car.

If you're alone and pay by check, you have corroborating evidence. If you pay in cash, don't leave the register tape in one of the grocery bags; take it with you as you go for the car. Remember which clerk checked you out. Even if you plan to load your own groceries into your car, make sure an identifying number is on your cart so that the bag boy can challenge anybody who tries to unload it without presenting a matching number. Report missing groceries at once, before a shift change takes away your chance to get corroborating testimony from the clerk who checked you out. And, finally, after you set out for your car, don't stop to gab with a friend or linger for any other reason. Get to the car and back to the groceries as quickly as you can.

I called Black and asked which store had made good on his stolen purchases. He said it was the Giant store at Old Georgetown Road and Democracy Boulevard.

I'm glad Black encountered a sympathetic manager, but I'd guess that others whose groceries have been stolen have been given short shrift by store managers. A manager who has heard this tale of woe several times before may very well become suspicious and say, "How do I know you didn't arrange for a friend to take your groceries -- and now you want me to give you a duplicate order free?"

It's a pretty tough judgment call for a manager to make, and I suppose in each case the decision must be based on many factors, especially the basic question: "Is this person known to me, or to my help? Is he or she a regular customer here?"

In all the foregoing palaver, I have not even mentioned one basic question:

What are you supposed to do when there's no bag boy on duty?

I think you'll agree that it's easier to try to protect your purchases than to talk a suspicious manager into making good on them.