It depends on what was lost on your detox -- water (or other fluids), fat, or muscle? Remember that when your scale goes down and says you lost "weight", you can't tell the cause -- is it because you lost fat? (a good thing!) or muscle? (a bad thing) or water? (no big deal).
As dieters, our goal is to lose body fat. But many diets promote a big water whoosh in the first weeks in order to get us excited about the scale going down. It's possible to lose an enormous quantity of water quickly when you cut carbs, because every gram of carbs you eat latches on to 3 - 4 grams of water. So when you suddenly cut out most carbs, you get a fast and large water dump and the scale goes down.
Water weight loss is meaningless though, and it doesn't change our overall health, body composition, or how our clothes fit. 60% of your body weight is water and it's going to vary from day to day, depending on your hormones and what you're eating. I know it's hard not to get hung up on those day-to-day scale fluctuations, but they're almost always changes in water retention and don't affect our fat loss efforts.
And yes, as soon as you start eating carbs again, the water will come back. But cutting carbs to dump water is still the oldest dieting trick in the book. Almost every diet plan out there will have you cut carbs in the first weeks to convince you that their plan works.
If the weight you've lost is muscle, that's a bad thing because muscle is your metabolic furnace and what burns calories in your body. The more muscle you have, the higher your metabolic rate will be and the more calories you will be able to eat and still lose. I don't know about you, but I like to eat!
But it's easy to lose muscle when we diet if we aren't exercising specifically to preserve and build muscle. Studies show that up to 40% of the weight lost with a traditional diet with no exercise will be muscle mass, not fat. And the effect is to lower metabolism and make it harder to lose fat. The only way to prevent this muscle loss is to lift weights/strength train while you're losing in order to preserve your muscle mass.
It's the yo-yo dieter's dilemma: you lose "weight", a good chunk of which is muscle, and then you go off the diet and gain back 100% fat. The end result is a changed body composition (less muscle and more fat) and a slowed metabolism from loss of muscle. Less muscle mass makes it even harder and slower to lose fat with each diet attempt. I yo-yo dieted my way to 57% body fat in 2001, which is truly a shockingly high number!
If what you lost isn't water and isn't muscle, that leaves us a third possibility: that what you lost is fat. And that's our goal -- to lose stored body fat! It takes a 3500 calorie deficit in order to lose a pound of fat, so you won't see as rapid of a fat loss as you will a water loss. But the only way you'll regain a pound of fat is to eat a 3500 calorie surplus. It wouldn't happen just by reintroducing grains into your diet, so long as your calorie count isn't excessive.
So back to your question - if you eat vegetable soup all weekend and lose some "weight", will it come back when you start eating normally again? I'd say yes, because most of that loss would be water from cutting back on carbs. You would lose some fat, but not pounds worth because you can't create a 3500 or 7000 calorie deficit over a weekend (enough to equal one or two pounds of fat). And you wouldn't lose muscle that quickly. So most of it would have to be water weight.
Hope that helps!