If you eat something once a week, you will probably **** it out. If you eat something semi-regularly but train hard, you will be ok. If you eat something regularly and everything else is equal, you will gain weight. Don't worry about it and train hard, if you are gaining weight, eat less, its not a death sentence if you put 2lbs on, you can lose it as quick as you put it on.
3500 extra calories a day would be 10lb...pretty sure it's doable and I've done it before. Food doesn't just disappear when you overeat it..
If your eating habits return to normal and you're still training you will likely drop all of that weight inside a fortnight. My weight varies by 4-5 kgs each week (~9-10lbs), it just depends what my diets been like, how much training I've done, whether I took a dump that morning.