Assuming these are typical RC 2 or 4 stroke engines, the fuel contains some 18% oil give or take. Very little if any of the oil burns. Therefore it has to go somewhere. So what doesn't get blown out the exhaust tends to find the low places to collect. Inside the muffler typically gets a bit which you dump out by pointing the model up. And the bottom of the crank case is another popular spot which can leak out the front bearing if they're not sealed. If they are sealed and they leak you may need a new bearing. As long as the engine performs as it should and it's not gushing oil from these places then there's no real reason to worry. Some engines simply leak in more places than others. At least you know they're getting well lubricated.

Then again you never said how much oil you're finding in these places. Nor the make and model of the engines, how they're mounted (upright, inverted, etc.) and fuel being used.