Things like that may vary from country to country, though. In Austria, roundabouts do not have a special rule and therefore a car approaching the roundabout theoretically has the right of way. But pretty much any roundabout has a yield sign, I've never encountered one without a yield sign at the entrance. Doesn't mean that there can't be a roundabout that does not have one.
As you described, the default is to yield, and so the vehicle can always yield and signal its intention to do so by breaking slowly. And I agree, there may not always be a sign (a storm or driver could knock it over), so the default should always be to yield.
